This paper presents a comparative study of biofouling at three sites, experiencing different environmental conditions, within a power station cooling system between September 1982 and August 1983. Microfouling and macrofouling were studied independently through the analysis of dominant species, number of species (S), diversity (H’), and evenness (J‘). The degree of affinity among samples was calculated through Czekanowski's Index and a multivariate method was applied to analyse data on a three‐variable basis: sites, dominant species and time.