The climatological conditions that are favorable to thunderstorm cloud development in the coastal zone of French Guyana are investigated. The analysis reveals that both the thunderstorm occurrence and the mean monthly lightning frequency are correlated to the Intertropical Convergence Zone annual oscillation, thus increasing from March to August when the South-East trade-wind flux drives continental air masses towards French Guyana and decreasing from September to February when French Guyana is under the influence of maritime air masses driven by the North-East trade-wind. The study of the lightning frequency values, observed during individual thunderstorm events, as a function of the cloud-top height shows that these values are intermediate to the lightning frequency values deduced from the parameterizations proposed by Price and Rind (1992) for typical continental and maritime thunderstorm clouds of a similar height. This behavior suggests that the air masses in which thunderstorm clouds develop in the French Guyana coast are mixed air masses with intermediate characteristics. Further, the absence of correlation between the lightning frequency values and both the cloud-top heights and the corresponding convective available potential energy (CAPE) values indicates that these parameters cannot be used to predict lightning activity in these clouds. Most probably, the aerosol concentrations in the subcloud layer and consequently the cloud droplet concentrations play an important role in the development of lightning activity in the considered clouds. ¿ American Geophysical Union 1995