We report here the case of a patient (BB), suffering from a precocious evolving dementia with impaired arithmetic performance, who showed specific and theoretical pertinent dissociations in basic mental arithmetic. First, in a task involving production of answers to simple arithmetic problems, a strong dissociation was found among operation: while multiplication was severely impaired, addition was moderately and subtraction only slightly impaired. A second dissociation was found between problems potentially solvable by rules and the others, with the former being better preserved. Finally, in multiplication verification tasks, the rate and distribution of errors among problems were not different from those observed in the multiplication production task. This pattern of performance like the one presented by the patient RG (Dagenbach and McCloskey, 1992), suggests first that stored arithmetical fact representations are segregated by arithmetic operation and second that a distinction has to be drawn between arithmetical rules and arithmetical facts. Last, the parallelism of performance observed here in verification and production tasks suggests that the same deficit(s) is (are) responsible for errors in both tasks.