Gossypium tomentosum is a cotton species endemic to the Hawaiian Islands. We studied several aspects of its reproductive biology, including potential pollinators, floral biology, and diurnal and seasonal flowering phenology. Flower visitors were observed in G. tomentosum populations on O'ahu, Kaho'olawe, and Maui. Primary visitors were introduced species, honeybees and carpenter bees, both of which were pollinating the flowers. No native bee species were seen visiting flowers. In examining floral biology we found that in some cases 10% of flowers had styles that were as short as the anthers or were recurved toward the anthers. In the greenhouse, in the absence of pollinators, these flowers were the only ones that set fruit. Flowering of G. tomentosum commences in January and February, following the rainy season, peaks in May, and may continue into August and September. In one year, after higher than average precipitation during the rainy season, there was a greater abundance of flowering, and flowering persisted later into the year. Transgenic varieties of commercial cotton, G. hirsutum, are grown in Hawai'i and are interfertile with G. tomentosum. Honeybees and carpenter bees are also known pollinators of commercial cotton. Because these pollinators are long-distance foragers, we estimate that transgenic cotton fields would have to be greater than 10 km from a G. tomentosum population to prevent gene flow.