Study of factors influencing soil insecticide toxicity are needed to reduce negative impacts of these products on beneficial insects. To date, if high toxicity differences between different type of soils have been reported, there is few specific studies on soil parameters influence on selectivity of soil insecticides to beneficial arthropods. To assess the specific impact of organic matter, the relationship between bio-availability of a soil insecticide, carbosulfan [Sheriff 1 Gr], and its toxicity on a small Carabidae, Bembidion lampros (Herbst.) on a sand enriched with increasing quantities of organic matter was studied. In laboratory, adults of B. lampros were put on different substrate, made of sand or sand with addition of organic matter at 3, 6 and 9% w/w, and treated with carbosulfan applied as granule at the rate of 312.5, 625, 1250 and 6250 microg a.i./m corresponding respectively to 0.5, 1, 2 and 10% of the recommended field rate. Mortalities of B. lampros were assessed after 14 day of exposure. In parallel, the total carbosulfan residue (total extraction) and bioavailable fraction (CaCL2 aqueous extraction) were determined 48h after substrate treatments. According to the mortalities and bio-availability obtained, a dose--response relationship was calculated and compared with a reference relation dose--response obtained on sand, where the bio-availability of the product was considered as 100% of the amount of product applied. Carbosulfan was highly toxic on sand for B. lampros, with 100, 57 and 50% mortality at 10, 2 and 1% of the recommended field rate. When organic matter was added to the sand, the toxicity gradually decreased. This reduction in toxicity was more rapidly observed on sand + organic matter than on pure sand. The mortalities were strongly correlated with the bioavailability, indicating first that the organic matter is fixing an important part of the insecticide and secondly reduce its toxicity to beneficial arthropods. The results suggest that it could be possible, with further research, to predict toxicity of products in the field on different kind of soil according previous laboratory toxicity models and soil analysis. In conclusion, the organic matter influences strongly bio-availability of carbosulfan. This bioavailability was strongly correlated to toxicity to B. lampros. With the improvement of bioavailability determination and method validation, the assessment of pesticide bio-availability in the substrate could help to estimate the pesticide toxicity towards carabidae on different type of soils.