Our study aims to determine how different climatic variables influence bat activity and mortality at wind farms in Portugal. The study was conducted from March to October 2007 at a wind farm with 20 turbines located in Northern Portugal. Bat activity was determined by ground bounded acoustic sampling, while mortality was assessed through fatality searches around each turbine. Sampling occurred weekly and activity was measured the night before fatality search. The highest activity and mortality rates were from Nyctalus leisleri and Pipistrellus pipistrellus. The majority of activity and mortality (95% and 94% in that order) occurred from August to October and both were significantly correlated with wind speed, temperature and relative humidity; mortality also appeared to be influenced by wind direction. Our results show that it is possible to establish a relationship between ground bounded activity and mortality. Our results are relevant for the implementation of effective minimization measures and, therefore, for bat conservation in the Mediterranean region. Specifically, our results show that nearly all (94%) of bat mortality at wind farms happens from August to October, at temperatures higher than 13.O°C, and wind speeds lower than 5.0 m.s-1.