Spermatogenesis was studied in 56 shrews (Sorex araneus L.) from two chromosomal hybrid zones in Poland. The hybrid zones were formed between chromosome races that differed in Robertsonian metacentrics. Shrews were compared in four classes: homozygotes, simple Robertsonian heterozygotes, complex heterozygotes forming four-element rings in meiosis I, and complex heterozygotes forming four- or five-element chains. There was a significant effect of karyotype on the level of germ-cell death and chain-forming complex heterozygotes suffered the greatest germ-cell loss. However, the estimated level of germ-cell death is probably insufficient to influence the fertility of these males.