Forty two isolates of Pythium spp. obtained from a single cucumber greenhouse from Batinah region in Oman were tested for their phylogenetic and phenotypic diversity. All isolates were identified to the species level using sequences of the Internal Transcribed Spacer (ITS) region of the ribosomal DNA. All isolates were found to be Pythium aphanidermatum. The length of the ITS region was 777 base pairs for 41 isolates and 778 bp for one isolate. Metalaxyl sensitivity tests showed high levels of variation in sensitivity to metalaxyl among Pythium isolates from the same greenhouse, where the EC50 values ranged from 0.07 to over 1.80, with an average of 0.83 μg ml -1. This appears to be the first report of existence of Pythium isolates with moderate levels of resistance to metalaxyl in Oman. Most isolates were found to be very aggressive on cucumber seedlings, causing over 70% mortality. This study reports phylogenetic and phenotypic diversity among Pythium isolates from the same greenhouse. Findings are discussed in terms of sources of these isolates and possible management.