Effects of chemical and physical (host macrophyte) parameters on the epiphytic diatom (Bacillariophyceae) flora in Shingle Bay Bog, Michigan were evaluated. Samples of epiphytes and water for chemical analysis were obtained at seven sites from July through August of 1980. A characteristic acid bog flora was observed at all sites sampled, with a total of 251 different taxa represented. Correlation analysis of cluster patterns of chemical data and cluster patterns of diatom communities show significant association. Principle Component Analysis indicated pH and associated parameters of hardness, dissolved oxygen, and alkalinity were the major factors in chemical variation. Evaluation of diversity (H’) measurements indicate differences in chemistry within a similar habitat and differences in macrophyte host had no effect on diversity. Evaluation of between-sample similarity (SIMI) indicated host macrophyte had no effect on epiphytic community structure, but differences in SIMI were associated with chemical variation.