The development of larvae of Copidosoma floridanum was studied using the noctuid Trichoplusia ni as a host. The number of precocious larvae present in each host stadium differed for female and male broods. In in vivo choice tests, precocious larvae contacted gonadal tissue in parasitised T. ni more often than eggs of the parasitoid Microplitis demolitor. However, the average duration of contacts was greater for parasitoid eggs than for host gonads, with 50% of eggs of M. demolitor being killed during 30 min. In competition experiments with larvae of both parasitoid species, 20% of contests resulted in death of both larvae within 30 min. none