The wild-type eye color of the horn fly, Hematobia iritans (L.) (Diptera: Muscidae), is a dark reddish brown. An apparent spontaneous mutation in a single adult colony fly resulted in a white-eyed mutant. A colony of white-eyed horn flies was established from this single individual and has been maintained in the laboratory. Laboratory crosses determined that the white-eyed phenotype is inherited as a simple Mendelian autosomal recessive with complete penetrance. No other differences from the wild-type flies were detected in the external characteristics of the mutant phenotype or in egg viability. However, white-eyed flies had significantly lower amounts of the pigment dihydroxyxanthommatin in their heads, suggesting either the lack of xanthommatin production, or a failure of transport and storage within the head of the mutant phenotype.