Male spider mites (Acari: Tetranychidae) guard preimaginal quiescent females to reserve paternity. In a few species of Stigmaeopsis that make densely woven silky nests on the surface of host plants, mature males attack and even kill rivals to gain their rights to inseminate young females in the nest. Here we report that the adult males of Schizotetranychus brevisetosus (Acari: Tetranychidae), whose web nest construction behavior evolved independently from Stigmaeopsis spp., also exhibit highly aggressive behavior towards one another. Combat mortality was measured in an experimental design where two males in the treatment group were forced to live in a nest made by adult females on a leaf arena, while one male in the control group was allowed to live in a nest on another arena. We found that the 5-day mortality of males in the treatment group (56 and 37 % in two replicates) was significantly higher than in the control group (9 and 12 %) and that lethal combats frequently arose in the treatment group. However, we found no consistent trend for the length of the first legs, which is known to affect the outcome of battle in Tetranychus urticae and Stigmaeopsis miscanthi. This might be due to a relatively small number of measurable specimens, and the effects of body size should be explored in future studies.