A statistical analysis of a 25 years time series of routine ozone soundings at Uccle (51¿48'N,4¿21'E) reveals that the frequency of tropopause folds is, within the limits of uncertainty, similar for all seasons. On the average, 4.8% of the ozone profiles show the presence of a fold whereas a shorter time series of soundings (1985--1993) obtained at the Observatoire de Haute Provence (43¿54'N,5¿45'E) yields only a 2% fraction of folds. The difference between both stations is however not entirely statistically significant. The average wet and dry &bgr; artificial radioactivity deposition in the sixties and early seventies was at a maximum at Uccle around June, in good agreement with global Sr-90 wet deposition measurements (Volchok and Kleinman, 1971) suggesting that stratosphere-troposphere exchange is more important during spring compared to the rest of the year. This leads to the hypothesis that, if tropopause folds are the major agent of stratosphere-troposphere exchange (STE), they may be more efficient in spring than in autumn. ¿ American Geophysical Union 1996