As part of a comprehensive study of the population dynamics of the whirligig beetle Gyrinus marinus Gyll. experiments were performed concerning flight activity. From capture-recapture experiments it appears that only a small percentage of the beetles move from one pond to another by flight. Flight activity only occurs if the weather is favourable, i.e. if the air temperature is more than about 17° C and the wind is feeble, and probably only if the sun is shining. In the Netherlands the opportunity to fly is therefore very limited by weather conditions. Flight activity of males and females occurs from April till the middle of October, thus including the period of reproduction. During reproduction females fly, but to a lesser extent than males. The different degree of dispersal during and after reproduction corresponds with the different degree of dispersal of mothers (and their eggs) and offspring. The hypothesis that flight occurred simply because the weather was favourable for flight cannot be rejected. The possibility of randomly occuring flight dispersal and the significance of a small dispersal activity for population dynamics is analysed in a separate paper with the help of simulations (van der Eijk in press).