The butterfly fauna was monitored at six semi-natural grasslands in southeastern Sweden . The aim was to investigate phenology, inter-observer differences, variations within a day and variations due to weather conditions in recorded butterfly species richness and abundances using the line transect method. A total of 30111 butterflies belonging to 46 species were recorded. The results concerning the phenology highlighted the importance of using flexible dates when monitoring butterflies. The butterflies were affected by weather and the time of day for recording. When the overall cloudiness of the sky exceeded 50 %, the number of species showed a drastic fall unless the temperature was at least 22 °C . The butterfly numbers also showed a sharp decline at temperatures below 19 °C if the proportion of sunshine of the transect walk was below 80–85 %. M any butterfly species showed well-defined daily rythms of flight activity, and the results indicated that transect walks should be performed between -4.5 and +4 h from the time when the sun reached its highest point. No significant difference in the number of observed butterflies between different recorders were found. Power analyses showed that minimal detectable difference for species richness when having five sites would be approximately 20 %. With this knowledge, the interpretation of results from butterfly monitoring will be facilitated and efforts can be made to minimize the impact of different factors on the data by adjusting the guidelines for monitoring.