The behaviour of Suillia flies on basidiocarps of Hebeloma radicosum was studied in central Japan. Hebeloma radicosum is a peculiar fungus that fruits only on talpid mole middens, near the moles' nests. All insect species around selected basidiocarps of this mushroom species were observed, their behaviour studied, and identified in late October to early November 1992, and again briefly in 2001. During these periods, Suillia flies were seen on the basidiocarps, while other insect species were rarely observed. Usually one Suillia fly (male) occupied one basidiocarp and prevented other conspecifics from occupying the same basidiocarp. This appeared to be pre-mating territorial behaviour. Larval stages of Suillia are known to be mycophagous, and the territorial behaviour is presumably performed to defend oviposition sites. This is the first report from Asia on ecological aspects of Suillia flies.