Helminthosporium vicotoriae and H. carbonum were grown under fluorescent plus incandescent lights, or in darkness, with several different temperature regimes. There was little or no effect of light on toxin production by H. victoriae. Light-grown cultures of H. carbonum had significantly higher titres of host-specific toxin than did dark-grown cultures. Light had no significant effect on growth of either fungus. Maximum titres of host-specific toxins from both fungi were evident at the time maximum growth was reached. Minimum pH values in Fries modified medium occurred at the time of, or slightly before maximum level of toxin was reached.