Information is presented on three fungal–algal associations on intertidal marine algae. They comprise two members of the Ascomycotina, Lautitia danica parasitic on Chondrus crispus, and Mycosphaerella ascophylli, an obligate endophyte of Ascophyllum nodosum, and a member of the Basidiomycotina, Mycaureola dilseae, that is parasitic on Dilsea carnosa. Details are given of life cycles and seasonal periodicity. The reproductive cycles of Mycosphaerella ascophylli and Ascophyllum nodosum are synchronised, with the fungal sporocarps being confined to the algal receptacles. Lautitia danica occurs on cystocarpic Chondrus crispus throughout the year, with older fronds being more likely to have fungal sporocarps on them. Mycaureola dilseae infecting Dilsea carnosa was only found during September and October. The systemic nature of the association between Mycosphaerella ascophylli and Ascophyllum nodosum is compared with the apparently localized occurrence of Lautitia danica and Mycaureola dilseae. Key words: marine algicolous fungi, seasonality, Ascomycetes, Basidiomycetes.