Activity of brown and yellow unbanded shelled Cepaea nemoralis (L.) was observed in 23 outdoor cage experiments carried out at various weather conditions in Słupsk, northern Poland. The experiments were carried out at noon local time and lasted one hour; the activity of each snail was scored at 5 minute intervals. In the range of temperatures examined in this study, air humidity was the significant factor associated with differences in the activity of the morphs. At air humidity of approximately 90%, all snails remained active throughout the experiment, irrespective of the colour of the shell. At air humidity of over 70% brown snails remained active significantly longer than yellow ones. When humidity was below 70%, yellow snails remained active significantly longer than brown ones, and the onset of inactivity was earlier with decreasing air humidity. No consistent effect of the level of solar radiation, air temperature, or air humidity/temperature ratio was observed. Even though the differences in the time of remaining active were not large, they indicate that yellow and brown C. nemoralis are adapted to different climatic conditions.