Blocks of bovine incisor dentin, on the root canal surface of which a smear layer had been formed, were inoculated in vitro with two strains of Prevotella nigrescens, a wild sampled strain and a reference one (NCTC 9336). Half the blocks were pretreated with irrigating solutions: 6% citric acid for 5 min + 6.25% sodium hypochlorite for 10 min. They were compared with the other blocks simply rinsed in distilled water (i.e. the control samples). The bacteria adhering to the dentin surface after an incubation time of 3 h were counted by direct examination using a scanning electron microscope. The adhesion of P. nigrescens was less marked on all of the samples treated with irrigating solutions. Adherence was particularly significant in the case of the wild strain (F = 10.22). The latter was far more active than the reference strain (F = 35.82). The use of a chelating agent at the end of root canal preparation served to remove the smear layer and limited the attachment of P. nigrescens to the dentin.