The butterfly fauna was monitored in six semi-natural grasslands in southeastern Sweden. The aim was to evaluate monitoring criterias for wind, sunshine, temperature and time of day for butterfly species richness and abundances when using the line transect method. A total of 30111 butterflies belonging to 46 species were recorded.Data from this study suggests somewhat stricter criteria for temperature and sunshine than stated in the widely used "Pollards walk". A sharp decline in butterfly numbers were detected at temperatures below 19°C if the proportion of sunshine of the transect walk was below 80-85 %. No effect of wind speed, up to five on the Beaufort scale, on observed numbers of species or individuals were found. Several butterfly species showed welldefined diurnal rythms of flight activity, and the results indicated that transect walks can be performed between -4.5 and +4 h from the time when the sun reached its highest point. The results of this study can be used to adjust general criteria in national monitoring and also for detailed regional and local monitoring where it may be important to adjust for diurnal rhythm and weather related bias.