In seven experiments carried out to optimize the induction of a mitotic gynogenesis, 323 potential mitogenes were produced out of 36 spawns. After 8 months, 119 fish had reached a stage of development enabling sexing from exterior characteristics. Gel electrophoresis using polymorphic serum protein, and enzyme systems (ADA, ALB, TF) as genetic markers confirmed their homozygous gynogenetic status. The overall sex ratio of 77 females to 42 males showed an unexpectedly high percentage of males (35.3 %) among the mitotic gynogenetic O. niloticus produced in our experiments. However, various sex ratios were observed among the mitogenes produced from batches of different females (percentage of males 0-100 %). In contrast to the sex ratio of mitogenes, only females were observed among 163 meiotic gynogenetic O. niloticus. The electrophoretic analysis evidenced high crossing-over rates for the loci, used as genetic markers.