Isolated polynucleotides encoding Brassica napus diacylglycerol O-acyltransferase (DGAT) are provided. Also provided are the encoded DGAT polypeptides, transgenic plants which express DGAT, and methods for making such plants. The DGAT-encoding polynucleotides are useful for increasing triacylglycerol (TAG) synthesis, seed oil content, and oil quality in plants.