Macroconidia of two isolates of Fusarium sulphureum (F. sambucinum f. 6) formed conidial chlamydospores in distilled water. Essentially all of the conidia placed in the system produced chlamydospores within 4–6 days. Differentiation of conidial cells into chlamydospores depended upon the age and concentration of the conidia. Conidia from 30- to 38-day-old colonies grown on potato sucrose agar produced chlamydospores in distilled water but those from 5-day-old colonies did not complete the differentiation to chlamydospores. At low concentrations (500 to 1000 conidia/ml), conidia of all ages germinated in distilled water, but did not form chlamydospores. At higher concentrations germination was inhibited and conidial chlamydospores were produced. In contrast, conidia of F. oxysporum at similar concentrations in distilled water germinated and produced hyphal chlamydospores. In a three-salts solution using pregerminated conidia, F. oxysporum and one isolate of F. sulphureum produced hyphal chlamydospores, but a second isolate of F. sulphureum did not form such chlamydospores. The first change observed in conidia of F. sulphureum during differentiation in distilled water was a loss of cytoplasm, usually from the apical and basal cells. Also, electron microscopy revealed that these empty cells lost the inner layer of their walls. The conidial chlamydospores of F. sulphureum contain the dense cytoplasm and have the thick walls characteristic of other Fusarium chlamydospores.