We describe the mating behavior in the spermatheca-lacking theraphosid species Sickius longibulbi Soares & Camargo 1948. The behavior in captivity of nine pairs of S. longibulbi was videotaped and analyzed. The mating of this species presented an uncommon theraphosid pattern. There is little in the way of overt courtship by the male, the primary behavior seen being the male's use of legs I and II to touch the female's first pairs of legs and her chelicerae. Sometimes the male clasped one of the female's first pairs of legs, bringing her dose to him. While the female raised her body, the male clasped her fangs and held her tightly with his legs Ill wrapped around her prosoma. The male seemed to try to knock the female down, pushing her entire body until she lay on her dorsum. In this phase we observed the male biting the female on the sternum or on the leg joints. When the female fell, the male attempted to position himself at an angle of 90° from the female. These movements appear to demand a lot of energy, particularly because the female is not passive during the mating. Our findings suggest that copulating in this position is, for the male, more successful than adopting other positions because it allows his extremely long palpal bulbs to deposit more sperm in the female oviduct where -- since she lacks spermathecae -- she retains the sperm. We suggest that the further he reaches into the oviduct, the greater the chance that he will fertilize the female's eggs. [ABSTRACT FROM AUTHOR]