During 1996 to 2001, samples submitted to clinics from commercial greenhouses involved 11 species and two unidentified isolates of Pythium from 110 plant samples, five potting soil tests, and five tests of irrigation water. Pythium irregulare was found in 45% of the plant samples, four of the five water samples, and three of the five potting soils. Pythium aphanidermatum accounted for 29% of all plant but 77% of the poinsettia samples. The Pelargonium samples received were infected with P. aphanidermatum, P. dissotocum, P. heterothallicum, group F, P. irregulare, P. myriotylum, and P. ultimum. The base pair sequence of the ITS1, 5.8S, and ITS2 regions of ribosomal DNA effectively differentiated the species encountered. The ras-related protein gene sequence did not differentiate P. aphanidermatum, P. arrhenomanes, and P. deliense from one another. One isolate each of P. cylindrosporum, P. dissotocum, P. heterothallicum, P. splendens, and P. ultimum exhibited resistance to the phenylamide fungicide mefenoxam, an isomer of metalaxyl, while 38% of the P. aphanidermatum and 37% of the P. irregulare isolates were resistant.