Morphological studies conducted on axenic cultures of the endozoic gymnodinioid dinoflagellate Symbiodinium (= Gymnodinium) microadriaticum demonstrated that there are intrinsic variations in the dimensions of the recently divided, but as yet unseparated, vegetative cells (the two-cell stage). Characteristic dimensions of a given strain are maintained when such a strain is artificially introduced into a host other than that from which it was originally isolated. Ultrastructural observations illustrated that, in contrast to the algae in their hosts' tissues, the algae in culture produced a fibrous or granular `pellicle' approximately 0.2 $\mu $m thick. The algae in situ in Tridacna gigas also demonstrated the presence of a `pellicle', but this structure was reduced when compared to that produced by the same cells in culture. Many of the stages in the described life cycle of the alga were observed, with the exception of those stages involving gametogenesis and sexual reproduction. Two distinct types of zoospores were observed; those that contained an `accumulation body' in the epicone and those that did not. 1. Endozoic gymnodinioid dinoflagellates isolated from 17 different invertebrate hosts were cultured axenically in ASP-8A under identical conditions of light and temperature. 2. Analysis of the dimensions of the two-cell stages of these algae showed that they fall into two significantly distinct size categories. 3. Observations of the zoospores showed that those of `small' strains possessed an `accumulation body' in the epicone, while those of `large' strains did not. 4. Examination of the ultrastructure of the amphiesma of the algae in situ illustrated that this structure is greatly reduced. By contrast, the cells in culture produced a `pellicle' approximately 0.2 $\mu $m thick, within which binary fission, or the production of tetraspores or of zoospores may occur. 5. Transmission and scanning electron microscopy of the cells in culture illustrated that the algal varied with respect to surface architecture. Most cells were smooth, while cells from Zoanthus sociatus uniquely possessed tufts of hair-like fibres projecting from the surface of the amphiesma.