Soil algae are present in all kinds of soils, both arable and virgin, in large amounts and in great diversity. These photosynthetic microorganisms, which are concentrated in the top few centimeters of the soil profile, are organized in a community structure that varies depending on soil type, farming method and pesticide application. We first tested several extraction and conservation methods for soil algae, and a C-14 incorporation procedure to evaluate the photosynthetic activity of these micro-organisms. In a second study, we assessed the sensitivity to atrazine of soil microalgae from two corn fields managed by different agricultural practices (conventional vs. organic). Changes in indigenous diatom communities were monitored, together with photosynthetic tests performed on the whole algal community. Comparison of the data for the treated field with those for the reference, untreated organic cornfield showed that previous atrazine application in the conventional cornfield had changed the species composition of the soil diatom communities. Short-term ecotoxicological tests, using photosynthetic activity as endpoint, also showed that the communities that had developed under pesticide stress were more tolerant to further atrazine application than the control communities. The stress caused by major environmental disturbances, such as ground dressing, was not sufficient to mask the difference between the two crops, suggesting that telluric microalgae could be used as indicators of xenobiotic contamination in soils. These initial findings about using soil microalgal communities as bioindicators are promising. In addition, their photosynthetic activity, which reflects their sensitivity to xenobiotic compounds, seems to be a relevant bioindicator of soil contamination.