Abstract.For initiating the development of tetraploid breeding lines in rainbow trout,Oncorhynchus mykiss(Walbaum), using different populations as a genetic basis, four populations out of a total of 11 were selected according to gene marker differences. Egg batches resulting from single‐pair and group matings and representing purebred and crossbred progenies of these populations were exposed to a late pressure shock for inhibition of first mitosis. In six out of 26 treated egg batches tetraploid embryos were identified by chromosome examinations. In the case of successful ploidization single‐pair matings yielded exclusively tetraploid embryos, while group matings contained some diploids. Successful tetraploidization was dominated by mating partners from one population. In total 413 putative tetraploid fry were available for further rearing. Besides diploids, triploids obtained by thermally induced retention of the second polar body were kept as controls mainly for methods to identify genome manipulated trout. Survival rates of tetraploids were low and loss frequencies did not stabilize till the age of 9 months. Thirty‐three tetraploid fish, individually checked by measurements of DNA content, nuclear cell size and number of nucleoli per cell, were found at the age of 17 months. Body weights of survivors were similar for tetraploids and diploids throughout the whole testing proc
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