Variable susceptibility to mite-associated ulcerative dermatitis (MAUD) attributable to Myobia musculi infestation was studied in breeding stocks of sixteen inbred strains of mice. Statistical evaluation of 41 occurrences of lesion among 1,517 mice of the various strains indicated that significant differences in the frequency of MAUD were associated with differences in genetic background and in the H-2 type. These findings suggest that the occurrence of an ulcerative skin lesion in inbred mice exposed to M. musculi is controlled by at least two genes. Lesion susceptibility appears to be profoundly affected by a non-H-2 linked gene or gene combination shared by all C57Bl background strains. In addition, a major influence is exerted by one or more genes within the H-2 complex.