The effects of four cations, Ca2+, Mg2+, K+, and Fe3+, upon the production of zoosporangia by isolates of four species of Phytophthora in axenic culture have been investigated. A response surface design was used to examine main effects and possible interactions. Responses to Ca2+ and Fe3+ were strongly quadratic with the higher concentrations inhibiting sporangial production in P. cinnamomi and P. drechsleri isolates. Responses to Mg2+ and K+ were weaker and, in the case of magnesium, were linear rather than quadratic. There was no significant interaction between cations except in the case of P. cambivora where the interaction between Mg2+ and K+ was significant at the 5% level. For each cation, the optimum concentration for zoosporangial production was calculated. The processes of zoospore release and infection of plant material by P. drechsleri showed a strong linear and weak quadratic response to Ca2+, higher concentrations of Ca2+ favouring these processes. A strong quadratic response was obtained to Mg2+, K+, and Fe3+. Interaction between Ca2+ and Mg2+ was evident for all three host materials used; Mg2+ and K+ showed an interaction when Pinus radiata was used as the host material. The implications and limitations of these results are discussed.