In Sweden monitoring of bat fauna is important since this mammalian group is a significant part of the biodiversity in the landscape that requires ongoing consideration and precautions and a rich species occurrence of bats usually is an indication of environments with great biodiversity. There have been different recommendations on when it is most effective to monitor bats during the night. Furthermore, the recommendations to stop fieldwork at midnight, due to a perceived drop in activity thereafter, limit the time for fieldwork considerably. There is also varying result on which weather factors effect bats the most. By investigating occurrence pattern of different bat species in Östergötland, Sweden, and by recording different weather factors I could determine that the time of the night when most species were active was between 25% to 70% into the night. I also concluded that there was no decrease in activity around midnight and that there were two weather factors that had a major effect on bats: increasing temperature had a negative effect and increasing air pressure a positive effect. It was also concluded that weather factors effect species differently.