Recently in our wild stock of the monogenic blowfly Chrysomya rufifacies a recessive mutation “white” (w) causing white instead of red-brown eyes spontaneously appeared (Fig. 1). This marker gene enabled us to clarify the genetic basis of monogeny in this species. F1 offspring produced by reciprocal crossings between normal (+/+) and white-eyed (w/w) flies were phenotypically wildtype (Table 1). In F2 offspring of female-producing (thelygenic) and male-producing (arrhenogenic) F1 females wildtype and white-eyed flies appeared in the expected 3:1 ratio; in several crossings a slight deviation of this ratio indicated a reduced viability of the w/w individuals (Table 2). Examination of F2 progeny of thelygenic F1+/w females, which had received the w allele from their father, showed that most of the F2+/+ females were thelygenic, whereas most of the F2 w/w females were arrhenogenic; among F2+/w females thelygenic and arrhenogenic individuals occurred in almost equal numbers (Table 3). On the other hand, when F2 offspring of thelygenic F1+/w females which had inherited the w allele from their mother were tested, most of the F2+/+ females turned out to be arrhenogenic and most of the F2 w/w females thelygenic; among F2+/w females thelygenic and arrhenogenic flies also were found in almost equal frequencies (Table 4). the sex-linked inheritance of the factor w following from these results was also confirmed by an analysis of the progeny of thelygenic F1+/w females backcrossed with w/w males. Among the R1 offspring of F1+/w females, which had received the w allele from their father, the +/w females were predominantly thelygenic compared to their predominantly arrhenogenic w/w sisters (Table 5). Analysis of R1 progeny of F1+/w females, which had inherited the w allele from their mother, yielded reciprocal results (Table 6). This mode of incomplete sex-linkage of the mutation “white” observed in C. rufifacies (Figs. 2–5) supports the hypothesis that thelygenic females are heterozygous for a dominant female sex realizer (F') with predetermined sex-determining properties, and that arrhenogenic females as well as the males are homozygous for the recessive allele f (Fig. 6). The recombination frequency between F'/f and the w-Locus was calculated to be 12.72±0.72 per cent.