Adult Neometadidymozoon helicis and Nematobothrium spinneri were found coiled in host connective tissue without any capsule wall of parasite origin. Eggs of N. helicis and Didymozoon brevi- colle passed through the gut of a teleost without harm, suggesting predation as a possible mechanism for the release of eggs in these species. An alternative mechanism in N. helicis involved ulceration of the capsule wall and release of adults and eggs to the environment. The onset of ulceration was not dependent on temperature or on the sizes of the parasites, but may be associated with the host's spawning season. Larval didymozoids found in the intestinal wall of teleosts were ensheathed in a fine membrane of host connective tissue. When fed to other teleost species, one type from Favonigobius exquisitus burrowed into the gut wall and survived for at least a month in Sillago analis. Information on the biology of didymozoids is scarce, possibly because the large pelagic scombrids in which most didymozoids occur are difficult to keep in captivity. The occur- rence of several species of didymozoids in pelagic and benthic fishes off southeast Queensland permitted observations on three aspects of didymozoid biology-the structure of the didymozoid capsule, the mechanism for the release of eggs, and the location and in- fectivity of larval forms. All hosts were from Moreton Bay except