Human asexuality is defined as a lack of sexual attraction, and research suggests that it may be best conceptualized as a sexual orientation. Sexual fantasies are thought to be universally experienced and are often understood to represent true sexual desire more accurately than sexual behaviour. We investigated the relationship between asexuality, masturbation and sexual fantasy as part of a larger online study. Self-identified asexual individuals were compared to sexual individuals with and without low sexual desire. A total of 924 individuals (153 men, 533 women, and 238 individuals who did not respond to the query about sex) completed online questions asking about masturbation and sexual fantasy. Five hundred thirty four were classified in the asexual group, 87 met diagnostic criteria for hypoactive sexual desire disorder (HSDD), 78 met criteria for subthreshold HSDD without distress, and 187 were a sexual comparison group (i.e., identified as sexual, and had no reported difficulties in sexual desire or distress). Asexual individuals were significantly less likely to have masturbated in the past month and significantly more likely to report never having had a sexual fantasy. Specifically, 40% of asexual participants reported never having had a sexual fantasy compared to between 1% and 8% of participants in the sexual groups. Eleven percent of asexual individuals reported that their sexual fantasies did not involve other people, compared to 1.5% of all sexual individuals. Taken together, these findings suggest that there are notable differences in patterns of sexual fantasy between asexual individuals and sexual individuals with and without low sexual desire.