Parasitic diseases of the skin may be classified into: 1, those in which the definite etiologic factor has been isolated and identified (microsporon furfur of tinea versicolor, the trichophyton of tinea tonsurans and circinata, and the achorion Schoenleinii of favus), and 2, those in which no absolute proof has been given of the constant presence of vegetable micro-organisms, and yet which in appearance, clinical course, reaction to antiparasiticides, and other circumstances, seem to be of parasitic nature (pityriasis rosea, eczema marginatum, mycotic eczema, etc.). In this paper it is my purpose to deal only with the following conditions: pityriasis versicolor, pityriasis rosea, tinea circinata (localized and disseminated), eczema marginatum and mycotic eczema, and endeavor to show that while in most cases these admit of ready diagnosis there are times when differentiation from other important conditions must be made. No one test in my experience for the determination of the parasitic