We investigated concentrations of mycotoxins during the growth of four cultivars of forage maize (Zea mays L.) in Nasushiobara, Tochigi prefecture, and their distribution in ears of maize grown in Morioka, Iwate prefecture, Japan. In experiment 1, we measured concentrations of naturally occurring fumonisin, nivalenol, deoxynivalenol and zearalenone at progressive crop growth stages. Concentrations of fumonisin in stems + leaves remained very low or not detectable, but those in ears became detectable at 40 days after heading and increased rapidly after 50 days after heading (DAH) (fumonisin B₁+ B₂<3260 μg/kg; mean value at 50–74 days after heading). Concentrations varied widely within cultivars on the same day. Concentrations of nivalenol, deoxynivalenol and zearalenone in stems + leaves and in ears were low or not detectable throughout the experiment. In experiment 2, we collected three ears of each cultivar at the late yellow-ripe stage that showed extreme symptoms of Fusarium ear rot. Concentrations of fumonisin were extremely high in the upper half of ears in all cultivars (fumonisin B₁+ B₂18,000–25,900 μg/kg) but low in the lower half and bracts. Concentrations of nivalenol, deoxynivalenol and zearalenone were extremely low or not detectable. These results show that fumonisin concentrations in ears increased rapidly after 50 DAH, they were extremely high in ears of all cultivars with symptoms of Fusarium ear rot, and fumonisin was the most common contaminant. These results will help reduce mycotoxin contamination.