Some types of simple and logically possible syntactic rule never occur in human language grammars, leading to a distinction between grammatical and nongrammatical syntactic rules. Comparison of the neuroanatomical correlates underlying the acquisition of grammatical and nongrammatical rules can provide relevant evidence on the neural processes dedicated to language acquisition in a given developmental stage. Until present no direct evidence on the neural mechanisms subserving language acquisition at any developmental stage has been supplied. We used fMRI in investigating the acquisition of grammatical and nongrammatical rules in the specified sense in 14 healthy adults. Grammatical rules compared with nongrammatical rules specifically activated a left hemispheric network including Broca's area, as shown by direct comparisons between the two rule types. The selective role of Broca's area was further confirmed by time x condition interactions and by proficiency effects, in that higher proficiency in grammatical rule usage, but not in usage of nongrammatical rules, led to higher levels of activation in this area. These findings provide evidence for the neural mechanisms underlying language acquisition in adults.