Parallel polymorphisms for B chromosome and supernumerary segments on M6, S9and S10pairs have been detected in two Argentinian populations ofDichroplus elongatus, Raco (RA) and Tafi Viejo (TV) (Tucumán Province). All these polymorphisms seem to be temporally stable. In all samples of TV, it was observed a tendency towards an excess of the B + SS6class. SS6produces a total restriction of chiasma formation in proximal position in the involved pair in heterozygous and homozygous males. In TV a consistent dose effect of B chromosome on interstitial chiasma (Xi) frequency was detected in males. The Xifrequency vs. number of supernumeraries plot fits to a linear regression. In both populations, the individuals with B's showed a significant increase in macro- and microspermatid production. This effect is more marked at RA. The cytophotometric analysis of DNA content revealed that in both types of males, B and non B carriers, most macrospermatids are diploid suggesting a probably origin by cytokinesis blocking. However, spermatids with 3 to 5C contents in B carriers were also detected showing that other mechanism (i.e. cell fusion) may also account to explain the effects of B's on abnormal sperm formation.