The behavioral repertoire of the Puerto Rican anoles, Anolis cooki, A. cristatellus and A. monensis was studied in the laboratory. The display of A. cooki and A. monensis revealed great similarities; consisting of one head bob pattern highly stereotyped in its temporal sequencing and total duration, sometimes accompanied by non-stereotyped dewlap movements. A. cristatellus display behavior consisted of two distinct head bob patterns (Type A and Type B). The A display is strongly stereotyped and used in the assertion and courtship contexts, while B display is less stereotyped and appeared only in the challenge context. Similar display modifiers used during aggressive interactions were found in A. cooki, A. cristatellus, A. monensis and two other Puerto Rican anoles, A. evermanni and A. gundlachi. In all these species the modifiers were progressively added during prolonged aggressive encounters and seemed to indicate the increasing arousal of the combatants. A grading scale was designed to measure the level of aggression in conspecific and congeneric interactions. It was based on the different behaviors that the lizards used during their agonistic encounters. The scale assigned a numerical value to each behavior according to their order of appearance in the sequence of behaviors performed in the encounter. Thus, a numerical value was derived for the level of aggression exhibited in each interaction, and that was called the "aggression index" (AI). The AI of A. cooki and A. cristatellus conspecific interactions was significantly higher than the AI of their congeneric interactions. The high AI of the cooki-cristatellus interactions was not explained by Murray's (1971) "mistaken identity argument", but as a function of their wide overlap in resource requirements. / Master of Science