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The sex ratio of a diploid–polyploid complex of spined loaches (Cobitis; Cobitidae) from Lake Müggelsee (Germany) was 1:22 in favour of the females. The percentage of males increased during three years of investigation from 2.9% to 10.6%. Within the years, male proportion changed significantly between the different dates of sampling. In laboratory, 50 specimens were reared from eggs in slightly warmer and softer water than in the lake. Since sex ratio was 1:24 in this group, the influence of environmental sex determination, hermaphroditism or sex selective mortality in the lake was assumed to be low. Erythrocyte measurements on 59 wild-caught specimens revealed a diploid–polyploid ratio of 1:7.4. The dominance of the polyploid sperm parasite was taken as the main reason for the observed unbalanced sex ratio. The absolute host-dependence of the parasitic polyploids demands a lower threshold value for male percentage, which should have been reached in spined loaches from Lake Müggelsee.