Earlier studies have shown that different input factors can influence the acquisition of past tense inflection of verbs (e.g. Ragnarsdóttir, Simonsen & Plunkett, 1999). Phonological factors and frequency factors may both play a role, therefore, in the acquisition of past tense inflection by normally developing (ND) and specifically language impaired (SLI) children. The influence of these two factors, however, may differ across the two groups of children. This study investigated past tense inflection in Norwegian SLI children and compared their behavior with ND children.