ABSTRACT: Discrete patches of suitable habitat and the consequent aggregation of organisms with similar habitat requirements often intensify competition, particularly among conspecifics. Habitat differences may generate interactions between smaller orlarger individuals at variable stages of development in many fungi, plants, and colonial animals. Size-dependent competitive outcomes and effects on sexual reproductive allocation indicate that differences among discrete habitats have potentially profoundecological and evolutionary implications. In the northern Gulf of Mexico, the colonial hydroid Hydractinia [GM] colonizes hermit crab-occupied gastropod shells (= microhabitats), which vary greatly in size and morphology. Here, the relationshipbetween the gastropod assemblages available for H. [GM] colonization and the frequency of conspecific encounters and sexual status of those colonies involved is documented. Field-collected shells bore 1 to 3 colonies, and the number of colonies pershell increased with shell size. In contrast to previous studies, conspecific encounters were not limited to juvenile colonies. Sexually mature H. [GM] colonies generally were not distributed differently among shells with and without conspecifics.Moreover, mature colonies predominated conspecific encounters on large shell species exhibiting certain morphology. Inanimate structures, such as dock pilings and rocky surfaces, provide additional large surfaces for Hydractinia colonization insome areas. These results suggest that previous Hydractinia spp. studies represent only a subset of the diversity of ecologically relevant possibilities with respect to available substrata. Also, the characteristics of a given microhabitat (e.g.shell) affect competitive outcomes and sexual reproductive characters of its constituent competitors.