The cytotoxicity of maitotoxin (MTX) and okadaic acid (OA) was studied on three mammalian fibroblast cell lines. Neutral red uptake (NRU), which measures cell viability, and morphological alterations were selected as rapid suitable responses. NRU allowed a precise toxicity quantification while the observations of morphological damage revealed differences specific to MTX (cell blebbing) and OA (cell rounding). BHK21 C13 fibroblasts, although less sensitive to MTX than the other cell lines, were chosen since they gave stable information and a two-stage morphological response with OA ("square"-shaped cells, then round cells). When NRU and morphology alterations were studied with crude extracts of Gambierdiscus toxicus and Prorocentrum lima, responses were typical of the dominant toxins, MTX and OA or related toxins respectively. Applied to several dinoflagellate extracts, the two tests revealed no toxicity for Amphidinium carterae, Ostreopsis siamensis, O. ovata and Coolia monotis (from La Réunion) and toxicity for A. carterae and A. operculatum (from Saint Barthélémy). When toxic, A. carterae extracts showed blebbing similar to that caused by MTX. Morphology alterations caused by A. operculatum crude extracts, different from those corresponding to MTX or OA, were also observed.