To investigate trap response of Microtus townsendii we set dirty Longworth traps soiled with faeces, urine and other debris and clean Longworth traps washed in hot water in field plots. Voles entered dirty traps significantly more than clean traps. This result was more pronounced in new animals and in young animals. When all clean or all dirty traps were set on a field plot, a much larger proportion of the animals known to occur on an area were caught in dirty traps than were caught in clean traps. This effect was most pronounced in the summer and fall period and was strongly correlated with the presence of breeding females.