A Phytophthora pathogen of trees and shrubs previously designated Phytophthora sp. O-group is formally named as P. inundata sp. nov. P. inundata falls within the P. gonapodyides-P. megasperma major ITS Clade 6, its present nearest known relative being P. humicola. It has non-papillate sporangia, fairly large oogonia (average ca 40 μm) with thick walled oospores, amphigynous antheridia, a distinctive colony type, a high optimum temperature for growth of 28–30°C, fast growth at the optimum, and a high upper temperature limit for growth of ca 35–37°. A study of the breeding system of eight P. inundata isolates showed them to be classically heterothallic with A1 and A2 compatibility types. However some P. inundata A1×A2 combinations failed to mate even though the same isolates mated successfully with P. drechsleri testers. Others were ‘silent’ A1s or A2s, unable to produce their own gametangia but able to induce gametangial formation in the opposite sexual compatibility type of another species. This indicates a partial breakdown of the sexual mechanism in the species. Two isolates (one A1 and one A2) were unpredictably and chimaerically self-fertile, suggesting A1+A2 chromosomal heteroploidy. The association of P. inundata with ponds and rivers and with root and collar rots of trees and shrubs after flooding is discussed.