Rhynchophorus ferrugineus (Olivier 1790) (Coleoptera: Curculionidae) is becoming a serious problem in Mediterranean areas where it is well-adapted, and now is present even in the United States (California). The infestations are primarily in urban areas where chemical control is not advisable and million of Euros are spent to control it. The effects of the entomopathogenic nematode Steinernema carpocapsae (Nematoda: Steinernematidae) on mortality, growth, as well as the immune activity of R. ferrugineus larvae, were investigated. R. ferrugineus mortality exhibited a positive trend with the dosage and duration of exposure to S. carpocapsae. The median lethal dose and median lethal time, important to optimize the treatments, were calculated. S. carpocapsae also had a detrimental effect on R. ferrugineus weight. In vivo and in vitro effects of S. carpocapsae on the phagocytic responses of R. ferrugineus hemocytes also were recorded. S. carpocapsae was not encapsulated by R. ferrugineus hemocytes. After 24 h, the number of hemocytes recorded in treated larvae was reduced. To investigate the defensive abilities of R. ferrugineus humoral and cellular immune systems, specifically against the bacterium Xenorhabdus nematophila (Enterobacteraceae), the minimum inhibitory concentration that inhibits bacterial growth was measured. This is the first time that this technique is applied to entomopathogenic bacteria.