Throughout the range of the common shrew (Sorex araneus Linnaeus 1767), repeated Robertsonian fusion mutations have led to a karyotypic polymorphism dividing the species into chromosome races. Studies of fertility were undertaken in the male, both of homozygotes and of heterozygotes forming meiotic multivalents of varying complexity. Observations made at pachytene, diakinesis/metaphase I and metaphase II did not provide evidence for fertility impairment in homozygotes or simple heterozygotes. Males forming a chain of seven chromosomes during meiotic prophase I were produced through a program of captive breeding, and were brought to premature sexual maturity through photoperiod manipulation. In these more complex heterozygotes, incomplete pachytene pairing was frequently observed (68% of cells), germ cell death was found to be elevated (23%), and data from analysis of metaphase II spreads are indicative of an increase in nondisjunction above background of approximately 10%. At the interface between two chromosome races, a complex of clines of chromosome frequency can be found. Field studies determined the routes and widths of four such clines (ko, 8.6km; kg, 28.4km; no, 37.1km; pr, 47.9km), and concluded that the courses taken are entirely consistent with maintenance through a mechanism of heterozygote disadvantage. Estimates of disadvantage made from cline width were in close agreement with those derived from the fertility studies.