We investigated daily activity patterns of the long-nosed mouse (Oxymycterus nasutus) in a high-elevation grassland field in southern Brazil. We censused the animals every other month from June 2001 to May 2002, determining capture time with digital timing devices attached to traps. Activity patterns were markedly diurnal and did not differ between sex or age classes. Captures were more frequent just after sunrise and just before sunset. Daily activity was strongly influenced by ambient light, with animals showing seasonal changes in their activity time according to day length but with some activity at night. Nocturnal captures were positively related with night brightness measured in log lm/m² (logistic regression; odds ratio = 1.76, P < 0.001). Temperature positively influenced the probability of capture of O. nasutus during daylight hours (quadratic regression; r² = 0.76, d.f. = 11, P < 0.001), with the highest predicted capture probability at 3–6°C and a sharp reduction above 21°C. Although diurnal activity has been reported for other Oxymycterus species this is the 1st study to rigorously quantify diurnal activity of a neotropical rodent in the field. O. nasutus apparently showed a more strictly diurnal pattern than described for other congeners.