SUMMARYSeveral species ofCylindrocladium, comprising 58 isolates, were studied in culture to determine their trophic and sporulation responses to various nutrients and to light.Cylindrocladium avesiculatumand two isolates ofC. floridanumwere thiamine deficient, whereas all other species and isolates tested synthesized essential vitamins. All carbon sources tested were utilized with remarkable similarity by all isolates investigated. All nitrogen sources tested were utilized well, some more rapidly than others, by all isolates, although growth was greater and more rapid when casein hydrolysate was the nitrogen source than when KNO3was used. Production of conidia was not greatly influenced by any of the sporulating isolates on any of the carbon and nitrogen sources. Perithecia were not formed on ammonium sulfate (unless buffered near neutrality) or malt extract media. Most species sporulated asexually better when incubated under near-ultraviolet and blue radiation, exceptC. citriwhich responded to red and far-red radiation, andC. parvumwhich was not affected by light.Cylindrocladium floridanumandC. crotalariaerequired radiation for formation and development of perithecia. Perithecial initials were observed in dark-grown cultures ofC. floridanum.Production of microsclerotia by eight isolates occurred on all five natural media tested, glucose-malt agar being the poorest. Glucose-lima bean agar was an excellent medium for maximum mycelial growth, microsclerotial production, as well as for inducing numerous conidia and perithecia. Glucose-asparagine is recommended as the synthetic medium of choice for taxonomic work, because it provides excellent growth and sporulation of all species investigated. Conidial septation varied with the species but was not observed to be influenced by the medium. Morphology of the apical vesicle of the conidiophore was variable among species and often within the same isolate, thus rendering this characteristic untenable for delineating species.