A study was conducted to investigate the effect of salinity on pythium damping-off and tolerance to salinity among Pythium aphanidermatum isolates from different geographical origins, genetic backgrounds and saline/non-saline soils. Increasing irrigation-water salinity from 0·01 to 5 dS m -1 significantly increased mortality in cucumber seedlings inoculated with P. aphanidermatum and reduced dry weight of non-inoculated seedlings. In vitro tests in culture media amended with different concentrations of NaCl showed that growth of P. aphanidermatum, P. spinosum and P. splendens isolates was stimulated or unaffected at salinity levels stressful for cucumber (electrical conductivity = 5 dS m -1). Significant differences were found in tolerance to salinity among 47 P. aphanidermatum isolates obtained from different greenhouses. The concentration of NaCl required to reduce growth of P. aphanidermatum isolates by 50% varied from 23 to 62 dS m -1, with an average of 46 dS m -1, however, oospore production was more sensitive to salinity and no oospores were produced above 20 dS m -1. Analysis of tolerance of P. aphanidermatum isolates to salinity showed no relationship between tolerance to salinity and geographical origins or genetic data. Isolates of P. aphanidermatum from greenhouses with no salinity problems were as tolerant to salinity as isolates obtained from salinity-affected greenhouses, suggesting lack of evidence for ecological adaptation within P. aphanidermatum in Oman. Increased mortality in cucumber seedlings at higher salinity levels may imply a synergistic interaction between salinity stress and salinity-tolerant Pythium species on cucumber seedlings, resulting in greater seedling losses.