Fumonisins are a group of mycotoxins mainly produced by Fusarium verticillioides and occur predominantly in maize and maize-based feeds. The contamination of feedstuffs with these mycotoxins poses a serious health concerns to animals as well as human beings. The present study aimed to verify the mycoflora, water activity (a_w) and presence of fumonisin B₁ (FB₁) and fumonisin B₂ (FB₂) in complete mixed rations samples (roughage + grain mixture, vol/vol) intended to lactating cows in 9 dairy farms in the state of São Paulo. The roughage was constituted of corn silage. The grain mixture was constituted by corn grains, soybean bran and cotton seed and mineral mixture. Samples of the diets were taken directly from the troughs batch of 15 cows, on 2 consecutive days at intervals of 24 h and every 15 d with a pe­riod of 45 sampling days per farm. It was extracted incremental samples in 25 spots to sum up a compound sample of 5 kg, which was homogenized and subdivided in 4 subsamples of 1.25 kg each. The serial dilution technique and pour plate were used to isolate the fungi. Samples were analyzed to verify the presence of FB₁ and FB₂ using an immunoaffinity column to clean up and HPLC with fluorescence detection. The mycoflora analysis of 288 samples revealed the occurrence of the genera Aspergillus (20.09%), Fusarium (14.16%) and Penicillium (11.48%). Fusariumverticillioides (0.97%) and Fusarium proliferatum (0.30%) were the most isolated species of feed samples. Water activity ranged from 0.91 to 0.93. With regard to the number of colony forming units (cfu), isolates of Fusarium species ranged from 1.30 × 10³ to 1.37 × 10⁶. FB₁ (9.02%) was detected in 26 diet samples (n = 288) with concentrations ranging from 4.05 to 356.72 µg/kg with mean contamination of 11.0 ± 13.06 µg/kg. FB₂ (28.12%) was detected in 88 diet samples (n = 288) with contaminations ranging from 7.90 µg/kg to 1,635. 97 µg/kg with mean contamination of the 26.23 (±46.08) µg/kg. Four farms presented the highest number of samples contaminated by FB₂ (n = 74) when compared with FB₁. The potential contamination of FB₂ in corn samples requires further research.