Most cases of ringworm of the groin due to Epidermophyton floccosum occur sporadically. Familial and conjugal transmission have been observed by several authors,1,5,8,9,11,14 but Sabouraud13 already pointed out that in the case of conjugal transmission "la contagion n'est pas constante." Almost all larger epidemics of eczema marginatum Hebrae reported in the literature2-7,10,12,15,16 were found to be caused by Epidermophyton floccosum. This is surprising and in apparent contrast with the low contagiousness observed with infections due to Epidermophyton floccosum. The epidemics reported only involved men. All observations made in the course of these epidemics indicated exposure to an exogenous source of infection. All attempts to find the source of infection remained unsuccessful; however, affirming Sabouraud's despairing remark10 on a school epidemic in Paris "Dans ces cas, le mode de propagation reste obscur." In March, 1960, the Montreal City Health Department asked us to investigate 2