Penicillium polonicum, a common mold on dry-cured meat products, is able to produce verrucosidin, a potent neurotoxin. The ability of P. polonicum isolated from dry-cured ham to grow and produce verrucosidin from 4 to 40 degrees C at water activities (a(w)) of 0.99, 0.97, and 0.95 on malt extract agar (MEA) and a medium made up with meat extract, peptone, and agar (MPA) was evaluated. Verrucosidin was quantified by high-pressure liquid chromatography and mass spectrometry. P. polonicum was able to grow on MEA and MPA at all the a(w) values tested from 4 to 37 degrees C but not at 40 degrees C. The optimal environmental conditions for growth were 20 degrees C, 0.99 a(w) on MEA and 20 to 25 degrees C, 0.97 a(w) on MPA, but the highest amount of verrucosidin was obtained at 25 degrees C, 0.99 a(w) in both media. No direct correlation between extension of mold growth and verrucosidin production was found. Temperature appears to be the most important factor ruling mycelial growth, whereas verrucosidin accumulation is mostly influenced by a(w). However, analysis of variance of the data showed that there was a complex interaction among all the environmental factors (medium, temperature, and a(w)) that significantly (P < 0.0001) affected growth and verrucosidin production. The reduction of a(w) to intermediates values of 0.95 has a stronger effect on growth on MEA than on MPA. Given that the meat-based medium proved to be an appropriate substrate for the biosynthesis of verrucosidin by P. polonicum, the ability of this mold to produce the toxin on meat products should be established.