Two closely related species of the trichopteran genus Stenopsyche–S. marmorata and S. sauteri– are both widely distributed in Japan, and their larvae often co-occur in stony stream beds. The larvae make nests by spinning nets in spaces between stones. In this study, I examined the effects of the intruder–resident relationship, bodyweight difference, and species combination on contest outcome in laboratory experiments. In S. marmorata, bodyweight was revealed to be a better predictor of winning a contest than was residency. In S. sauteri, however, there was a prior-resident effect in the contests, and takeover occurred in a few cases when the intruders were larger than the residents. The duration of agonistic behavior increased as the bodyweight differences between the fighting individuals decreased. In heterospecific contests, outcomes were generally asymmetric: the larvae of S. marmorata (the larger species) rarely intruded into the nest of S. sauteri, while they, in most cases, protected their own nests from intrusion by S. sauteri larvae. In conclusion, the resident wins in both forms of competition when either S. marmorata or S. sauteri is an intruder.