An intensive cytological study was made of chromosome behavior at anaphase I in 12 intervarietal hybrids of cultivated rice. Two types of unusual behavior were found, bridges without fragments and bridges accompanied by acentric fragments. Since bridges without fragments were also found in each of 3 homozygous varieties in approximately the same frequency as in the hybrids, it was concluded that this behavior was not abnormal and did not indicate the presence of structural differences in chromosomes. Bridges accompanied by fragments were found at very low frequencies in 9 of the 12 hybrids, indicating the presence of paracentric inversions. These results provide proof of widespread structural differentiation in the chromosomes of cultivated rice varieties and suggest strongly that the partial sterility which occurs commonly in intervarietal hybrids is caused by complex inversions of the included type.