Abstract: One hundred and fourty nine crabs of the species Callinectes rathbunae Contreras parasitized with the rhizocephalan Loxothylacus texanus Boschma were captured. The average shell width was 8.48 cm. Three types of externa were registered in the hosts: virgin, immature and mature externa; their number varied from one to four, being more common the crabs with one externa. The behavior of parasitized crabs was registered during six months. The most aggressive crabs were those that presented virgin externa (n = 82), while the less aggressive ones were those who presented three to four mature externa (n = 3). The crabs with virgin and immature externa conserved the capacity of burying themselves, while the organisms with mature externa lost that capacity. The animals with mature externa passed most of the time grooming and oxygenating the externa they carried. The crabs with mature externa presented bigger voracity that the crabs with immature externa. It can be concluded that the level of aggressiveness of the parasitized crabs is related with the degree of development of the externa, the degree of aggressiveness of the parasitized crabs is not related with the sex and mature externa represent a great metabolic load for their hosts.