The object of the present study was to gain additional information about the genomic structure ofF. pratensis, F. arundinaceavar.glaucescens, F. arundinacea(2n= 42), F. Mairei and to confirm through chromosome pairing in these species and their interspecific hybrids the homologies suggested on karyotypic basis between their chromosome complements. Based on two sources of evidence (karyotype and meiosis of F1progeny) genomic formulae to different species are assigned.1.In the triploid hybrids involvingF. pratensis X F. arundinaceavar.glaucescens, chromosome pairing at M-I could be represented by a mean of 7.48I, 5.48II, 85III. In some cells as many as 19 chromosomes were associated.(a)This pairing is regarded as allosyndetic though some amount of intergenomic pairing was also observed.(b)Considering the karyotypic similarities between the satellited chromosomes ofF. pratensisand some of those ofglaucescens, and also the meiosis of their F1progeny, the genomic constitution ofpratensisandglaucescensmay be represented as AA and AA BB respectively.(c)In the tetraploidglaucescenspossibly a genetic mechanism exists which ordinarily suppresses intergenomic pairing (A-B).2.F. pratensiswas also crossed withF. arundinacea(2n=42). In the latter both the bred variety S.170 and the wild North African population Bn 274 were used.(a)In the crossF. arundinaceaBn 274 XF. pratensis, 23 of the 28 chromosomes paired and only 11.11% formed multivalents. The mean pairing at metaphase I was 4.88I, 10II, 59IIIand 33IV. In the other crossF. pratensisXF. arundinaceaS.170 hybrids had either 28 or 35 chromosomes. In the tetraploid hybrid 24 chromosomes paired and 18.55% formed multivalents. Thus in the latter cross there was a high percentage of multivalents.The pentaploid hybrid possibly had onepratensisgenome in excess.(b)In the tetraploid hybrid, pairing was mainly allosyndetic between sevenpratensisand sevenarundinaceachromosomes suggesting close homologies of their genomes. The remainder of the paired chromosomes were fromFestucawhich showed a variable amount of autosyndesis and partial homologies of the two genomes inarundinacea.This is further supported by the meiotic analysis of the pentaploid where there was an increase in the multivalent formation, especially trivalents.(c)The genomic constitution ofF. arundinacea(2n= 42) is suggested to be AA BB B'B'.(d)It is also suggested that there are residual homologies between the B and B' genomes ofarundinacea, the expression of which is suppressed at the normal meiosis of species. This is possibly accomplished either by strong preferential pairing, or is more likely to be due to some genetic control.3.In the F1progeny of the crossF. arundinaceavar.glaucescensandF. arundinacea(2n=42) about 28 of the 35 chromosomes associated with a mean of 7.0I, 10.9II, 1.3III, 5IVand 06Vper cell.(a)This pairing is interpreted as allosyndetic (A-A; B-B) though some amount of intergenomic pairing is also observed.(b)Structural hybridity between the A and B genomes of the two forms is suggested.4.A high amount of pairing among the chromosomes ofF. arundinaceavar.glaucescensXF. Maireihybrid (26.15 out of 28) suggests that the genomes in these two species are very closely related and possibly arose from the species which were genomically similar. The genomic formula forF. Maireiis suggested as A'A' BB.5.In the origin of theF. elatiorL. complexF. pratensisor a similar form is a likely donor of the A genome as the latter is common in all the forms includingF. arundinaceavar.glaucescensandF. arundinacea(2n= 42). In the origin of the hexaploid form probably three species were involved. The presentglaucescensdid not originate from the cross ofpratensisXarundinacea(2n= 42). BothF. arundinaceavar.glaucescensandF. arundinacea(2n=42) are suggested to be allo and segmental auto-allopolyploid forms respectively. In the latter the autoploid portion of B was segmental in character (B, B').6.A study of chromosome pairing in the octaploid hybrid between the decaploid and hexaploidarundinaceaforms revealed that 45 out of 56 chromosomes were associated as II to IV. The mean number of univalents per cell was 10.41 and only 10% of the chromosomes formed multivalents. Two alternative explanations are given to account for this:(a)The paired configurations were due to autosyndesis and allosyndesis and also that A and B genomes in the two forms have undergone differentiation.(b)Alternatively the decaploid had a genomic formula of AA A'A′ BB B′B′ B″B″ and pairing in the octaploid was exclusively due to allosyndesis.7.The origin of decaploidarundinaceafrom a tetraploid (glaucescens) and hexaploidarundinaceacross, followed by a fixation of gene mutations leading to a diploid-like meiotic behaviour of the decaploid, is discussed.