ABSTRACTThe nature and seasonal extent of microbial fouling onAscophyllum nodosum(L.) LeJol.,Fucus vesiculosusL.,Polysiphonia lanosa(L.) Tandy andChondrus crispusStackh. were observed by scanning electron microscopy. Bacterial filaments and smaller rod and coccoid forms dominated the fouling communities on all species, with pennate diatoms constituting a minor component in contrast to results with plastic substrates on which pennate diatoms dominated and preceded bacterial colonization. The total percent microbial coverage on the surfaces of all four seaweed species was determined by monthly stereological analyses of representative composite micrographs. These showed a simultaneous decline between April and May which could represent the die‐off of the cold water bacterial flora when water temperature increased past the threshold for obligate psychrophiles. Microbial colonization patterns were directly correlated (P = 0.005) with maximum coverage in April and November–December and reduced levels from May to October. Significant inverse (P<0.041) correlations between total percent coverage and water temperature indicate distinct seasonal cycles, however, the patterns of dominance by filamentous bacteria and rod and coccoid forms were markedly different. Total coverage patterns of both rhodophytes showed no apparent seasonal cycle and were not related to water temperature. Rod and coccoid bacteria were apparently suppressed year round onP. lanosarelative to the other species. These interspecific differences in seasonal fouling patterns are discussed in light of possible modes of regulation, especially algal antibio