The microorganisms inhabiting organic mulch applied in avocado orchards in Western Australia were studied to determine their role in the suppression of Phytophthora cinnamomi. Populations of aerobic bacteria, fungi and actinomycetes within the organic mulch changed with time. Temperature and moisture levels of the mulch were positively correlated with the microbial populations. Bacterial and actinomycete numbers tended to increase following incubation of the mulch infested with P. cinnamomi. The infectivity of P, cinnamomi after 3 weeks of incubation in the mulch was negatively correlated with populations of fungi and actinomycetes.