none Interspecific larval competition between the two aphid parasitoids Lysiphlebus testaceipes (Cresson) and Aphidius colemani Viereck was studied on two different hosts, Pentalonia nigronervosa (Coq.) and Aphis fabae Scop. Both species oviposited readily into heterospecific parasitized hosts if the time interval between the first and the second attack was less than 6 hours. Similar adult number of both parasitoid species emerged from multiparasitized aphids, indicating that neither parasitoid is intrinsic superior. The sequence of attacks (first attack either by L. testaceipes or A. colemani ) did not influence these findings. Since comparable results were found for both aphid species, interspecific larval competition does not oppose a simultaneous release of both parasitoid species in biocontrol programmes. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships; Ecology; Competition Aphidius colemani (Braconidae); Hemipteran hosts; Aphis fabae & Pentalonia nigronervosa; Parasite interspecific larval competition, influencing factors; Host preference; Aphidoidea (Hemiptera) parasitized by competitor species; Prevalence; Aphidoidea (Hemiptera), interspecific relationship between parasites Lysiphlebus testaceipes (Braconidae); Hemipteran hosts; Aphis fabae & Pentalonia nigronervosa; Parasite interspecific larval competition; Host preference; Aphidoidea (Hemiptera) parasitized by competitor species; Prevalence; Aphidoidea (Hemiptera), interspecific relationship between parasites; Parasite parasite relationships; Larval competition with Aphidius colemani in Aphidoidea (Hemiptera); Interspecific competition; Aphidius colemani; Larval competition within hemipteran hosts, influencing factors Aphis fabae; Pentalonia nigronervosa (Aphidoidea ); Hymenopteran parasites; Aphidius colemani & Lysiphlebus testaceipes; Parasite interspecific larval competition, influencing factors; Host preference; Prevalence; Aphidius & Lysiphlebus (Hymenoptera) interspecific relationship; Parasite parasite relationships; Larval competition in parasites Aphidius & Lysiphlebus (Hymenoptera)