Benthic dinoflagellates belonging to the genus Prorocentrum have a wide distribution, with Prorocentrum lima as the most commonly reported morphospecies.Some Prorocentrum species are Diarrheic Shellfish Poisoning (DSP) toxins producers.These toxins include okadaic acid (OA) and its derivatives, and dinophysistoxin (DTX)and derivatives. Most of these toxins are widely known as phosphatase inhibitors (PP) and potent tumor promoters. It has been proposed that the interaction between bacteria and P.lima has a great impact on the physiology of both microorganisms, and also in their environment. However, the ecological significance of bacterial-dinoflagellate relationships remains unclear. Therefore, the aim of the present study was to examine the role of dinoflagellate-associated bacteria in the toxin production of P. lima. Toxin profiles of Prorocentrum isolates from various localities along the Mexican coast were analyzed by LC-MS/MS. Cell extracts were prepared from five selected isolated and assayed for bioactivity against Staphylococcus sp. and Vibrio sp. In addition,bacterized monoclonal cultures of the dinoflagellates were treated with antibiotics to evaluate the physiological response on their volume, growth rate and toxin production over time. Six main toxin analogues were detected from Prorocentrum cultures; from these, OA (>50%) and OA-D8 were the most abundant analogs. The results showed differences in toxin composition of the isolates: the ones from Veracruz had a more diverse toxin composition, and isolates from Quintana Roo and Baja California Sur showed more conservative toxin profiles and less richness in analogs. This may be related to the accelerated nufcrient inputs to the Veracmz port coastal waters, where an increase in harmful algal blooms (HABs) has been observed. As well, the similarities in the toxin profiles of isolates from Quintana Roo and Baja California Sur, could suggest a low genetic variability, however, these results have to be confirmed by molecular analysis of clones from both areas.The bioactivity assay against Staphylococcus sp. and Vibrio sp. showed no effect from the extracts repared from the cells or bulk media of Prorocentrum. These results suggest that there is possibly no allelochemical effect between Prorocentrum and the bacteria tested. However, it would be important to complement this assay with additionalexperiments using axenic cultures of -P. lima to evaluate changes on its growth cycle and toxin production, when P. lima is cultured just with a specific consortium of bacteria. Regarding the time series experiment, maximum growth rates of 0.22, 0.35 and 0.17 div day"1 were found for the antibiotic, washed and non-treated culture, respectively. Additionally, no significant correlation was observed among P. lima cell volume, bacteria cell counts, and total toxin concentration. Bacterial cell density from the antibiotic treated culture was reduced only until day 16, therefore an axenic culture was not stablished. However, a positive correlation between the increase in bacteria and P. lima cell density was detected. These results indicate that the concentration of antibiotics used in this experiment did not affect the physiology of P. lima. Thus, the antibiotic doses used in this experiment were just as effective as only preliminarily washing the cells to reduce bacterial concentration. The results presented in this study so far demonstrate that toxin profile, composition of each toxin and biovolume of P. lima do not change during the growth cycle. In addition,the antimicrobial agents added to the growth media did not alter growth or biovolume. In conclusion, even though this study did not accomplish to explain in deep the interactions among bacteria associated to P. lima and their effect on the toxin production of the dinoflagellate, these results are a basis for a better understating of the physiology of P. lima and the importance of getting axenic cultures to solve this question.