Triploid rainbow trout produced by heat shock and control (diploid) siblings were raised separately at similar density, feeding and water quality regimes. No significant differences in body weight or condition factors were observed; however the weight of the eviscerated carcass was on average 20% higher (P<0.05) in triploid fish compared to diploid fish at 20 and 44 months post-hatching. The effects of triploidy on males were most evident during the final stages of spermatogenesis; in contrast, the gonadal development of triploid females was affected during its early stages, with the majority of the oogonia (30-70%) remaining within the oogonial clusters. A major finding was the presence of male-differentiating areas in most triploid females examined, which by the end of the sampling period appeared as gonadal hermaphrodites. Testicular weight, gonado-somatic index, sperm cell density and spermatozoa motility were significantly lower in triploid than in diploid male siblings, although some triploid males produced viable progeny when crossed to normal (diploid) females. Characterisation of this progeny by image analysis of nuclear DNA revealed the presence of a near-triploid genome. A single 5 month-old juvenile had developed testes in meiotic phase, providing a first evidence for the generation of limited numbers of viable progeny by autotriploid rainbow trout males. A cytogenetic analysis was carried out on monosex diploid and triploid populations of Nile tilapia. Synaptonemal complex analysis in diploid genotypes revealed the presence of an incompletely paired segment in the terminal region of the longest bivalent in heterogametic (XY) genotypes, which was not observed in homogametic genotypes. This unpaired region provides cytological evidence for the chromosomal basis of sex determination in O. niloticus. Meiotic analysis in triploids revealed the presence of longer (P<0.0001) synaptonemal complexes in heterogametic (XXY) than in homogametic (XXX) genotypes, with a significantly different (P<0.0001) nature of pairing evident between both groups. A model to explain the different progress in gametogenesis observed between male and female teleosts is discussed.