Pathogenic strains of Phytophthora infestans insensitive to phenylamide have been reported from around the world and are responsible for a lack of late blight control in both potatoes and tomatoes. In vivo laboratory assay methods used to determine the sensitivity of P. infestans to fungicides include floating of leaflet disks on fungicide suspension and the use of potato tuber disks. In our study, these two methods were compared with a new detached-leaf assay. The fungicides chlorothalonil (protective), oxadixyl and metalaxyl (systemic), and cymoxanil (locally systemic) were utilized. Phenylamide-insensitive and -sensitive genotypes of P. infestans were used as inoculum. Significant differences between preventive and systemic fungicides were found, and all assay methods differentiated between phenylamide-sensitive and -insensitive genotypes of P. infestans. Differences among assays were significant when comparing mean inhibitory values. There was no significant difference between the detached leaf and the tuber disk assay, but both methods were significantly different from the floating-leaflet disk assay.