A comparison was made between the absolute and actual fecundity of C. taenia and allotriploid Cobitis females from a diploid-polyploid population inhabiting the Bug River, of Vistula R iver basin in Poland. All specimens were measured, weighed and aged. The absolute fecundity was determined by a gravimetric method using 29 ovaries (three of C. taenia, 25 of triploids and one of tetraploid). The actual fecundity was estimated according to the method adopted by Halačka et al. (2000) and used five ovaries of C. taenia and 40 of Cobitis triploids. Absolute fecundity of C. taenia under analysis ranged from 1 819 to 3 302 eggs and equaled 2 487 on average. It was significantly lower than absolute fecundity of an exclusively diploid population in Klawój Lake described previously (Juchno & Boroń 2006a). Absolute fecundity of triploid females ranged from 285 to 3 710, with an average of 1 577 eggs, whereas the fecundity of tetraploid Cobitis female was low, with only 882 eggs. The highest actual fecundity of C. taenia as well as of triploids was observed after they laid their first batch of eggs.