Chromosome numbers of 2n= 13, XO and 2n= 17, XO are reported for 2 genera of the Alydidae. Meiotic behaviour is comparable to that of species in 6 genera of the Coreidae which also have an XO sex determination mechanism in males, and chromosome numbers of 2n= 15, 19 and 21. A further 3 coreid genera in which 2n= 18, 20, 22 and 26 are found have an XnX2O mechanism, the meiotic behaviour of which differs in some respects from published reports of other Heteropterans with multiple X chromosomes. The large m-chromosomes present in several species have permitted their morphology to be determined. The significance of achiasmate pairing of m-chromosomes and monhomologous X chromosomes in achieving regular segregation is noted, and the role of chromosome number changes in the evolution of the Coreidae is discussed.