Blastocystis sp. was detected in faecal samples from domestic dogs and cats in Brisbane, Australia. The prevalence rates were high, with 70.8% of the dogs and 67.3% of the cats infected with this organism. Blastocystis sp. from faecal material from two dogs was successfully cultured on inspissated egg slant medium for several months, but could not be maintained for longer periods. Blastocystis sp. from feline faecal samples failed to grow in culture. The parasites found in dogs and cats were generally smaller than Blastocystis hominis from human faecal material, and were the vacuolar form rather than the multivacuolar form. Otherwise, the general morphology of these organisms appeared similar to B. hominis when examined by light and transmission electron microscopy.