The influence of low frequency ultrasounds (28 and 40 kHz) versus mechanical stirring on the transesterification reaction of neat vegetable oil with methanol under base-catalysis was studied and the results are presented. It was found that the optimized variables of 6:1 methanol/oil (mol/mol), 0.5% NaOH (wt/wt), and 40 kHz ultrasonic irradiation at 25 degreesC for 20 min gave a maximum isolated ester yield of 98%.