Fish abundance, fish length and species diversity were measured on 24 artificial reefs of 3 sizes over 2 years in a northern temperate lake. Reefs were randomly ordered in a linear array in a relatively homogeneous environment at a depth of 3.1 meters. Half of the reefs were "seeded" by addition of fish. Assemblages were compared with natural assemblages occurring on the unseeded reefs to determine the influence of chance on species assembly. Seeded and un-sseded reefs were similar in seasonal fish abundance, fish length and species diversity. Fish abundance and length were largest on the largest reefs, implying habitat size affects fish abundance and size distribution. Patterns of fish abundance and length were found to be consistent between years for a given season. Summer (May through August) was divided into three approximately equal periods. The fall (September and October) was not divided. Measures of fish abundance and length varied between summer and fall seasons, which was probably due to offshore migration of fish to winter refugia. Deterministic (predictable) factors such as competition for limiting food resources and habitat size appear to be of primary importance in structuring seasonal lake fish assemblages at a local scale in this system, probably associated with in-shore summer and off-shore winter migrations to and from refugia.