Rhynchosporium alismatis, a pathogen of several Alismataceae species, is being studied in Australia as a potential weed biocontrol agent for Alisma lanceolatum, A. plantago-aquatica and Damasonium minus. Chlamydospores of R. alismatis are described for the first time. Large numbers (5.34×105 cm−2) were produced on potato dextrose agar within 8–15 d and range in size from 4.7 to 14 μm. There was variability between isolates in mean diameter but size was not related to the host of origin. Within 24 h at 30 °C 60% of chlamydospores, 8 d to 3 months old, germinated. Each germinated chlamydospore produced up to four germ-tubes. Chlamydospores were detected in leaf lesions on naturally-infected A. plantago-aquatica. One-month-old chlamydospores were pathogenic to leaf discs cut from mature leaves of A. lanceolatum and Damasonium minus. The relative robustness of chlamydospores is an advantage over conidia for the future production of a biological control agent if they can be produced and harvested in broth culture.