The energetics and behavior of the parapodial-swimming Aplysia brasiliana were investigated in order to compare net cost of transport (COTnet) between swimming and crawling, and to compare transport costs with other swimmers. Oxygen consumption (VO2) increased with increasing animal mass for resting, crawling, and swimming animals. Slopes of the regressions of log VO2 on log mass were 0.90, 0.91, and 0.89 for resting, crawling, and swimming, respectively. The regression for resting VO2 on mass was significantly lower than regressions of crawling and swimming on mass, which fell into a statistically homogenous subgroup. During 4-h swimming bouts, parapodial beat frequency dropped by less than 10% of starting values after 2 h and then stabilized for the remainder of the trial, whereas velocity steadily decreased to about 70% of starting values over the 4-h period. Initial beat frequency (at the start of a swimming bout) was negatively related to body mass, varying from 1.1 beat s− 1 for a 34 g individual to 0.7 beats s− 1 for a 500 g individual. Final beat frequency (at the end of a swimming bout) was also negatively related to body mass, but had a significantly lower intercept than initial beat frequency. Neither initial swimming velocity nor final swimming velocity was related to mass, but final velocity was significantly lower than initial velocity. A 250 g A. brasiliana swam at 345 m h− 1 and crawled at 7 m h− 1. Swimming COTnet (0.1 ml O2 kg− 1 m− 1) for a 250 g A. brasiliana was 50 times less than crawling COTnet (5.3 ml O2 kg− 1 m− 1). While the crawling COTnet for A. brasiliana fell within the range of other marine gastropods, swimming COTnet was less than that of swimming crustaceans, and much less than another gastropod, Melibe leonina, that uses lateral bending to swim.