Commercial concentrated and diluted (1:1, 1:2, 1:3; juice: water) steamed grapefruit juice was inoculated with known aflatoxigenic aspergilli and sampled after 10 and 14 days of incubation at 28 C. When Aspergillus flavus grew in juice, most aflatoxin B1 (0.211 μg/ml appeared in concentrated juice and least (0.013 μg/ml) in single strength juice. Juices diluted 1:1 and 1:2 yielded 0.078 and 0.020 μg B1/ml, respectively. Results were more striking when Aspergillus parasiticus grew in samples of juice. After 10 days, amounts of aflatoxin B1 in concentrated juice and in concentrated juice diluted 1:1, 1:2, and 1:3 were 7.5, 1.59, 0.69, and 0.56 μg/ml, respectively. Aflatoxins B2, G1, and G2 were also produced and greatest amounts also developed in concentrated juice. Amounts of these toxins decreased markedly when the percentage of soluble solids in the juices decreased. Fourteen instead of 10 days of incubation resulted in increases in the amount of each toxin in concentrated juice and in concentrated juice diluted 1:1. Although the greatest amount of aflatoxin occurred in concentrated juice, appearance of visible growth and onset of sporulation by the molds was slower in this than in diluted juices. The pH of the concentrated juice did not change appreciably after 10 and 14 days of incubation, but the pH of diluted juices rose progressively from the initial value as the percent soluble solids content in the juice decreased.