Form-priming effects occur when the prime has similar but nonidentical orthographic form to the target (e.g., mature-NATURE). In visual word recognition tasks, strong facilitatory effects of form similarity are readily obtained when the prime is heavily masked and cannot be reported. In the lexical decision task, this effect is subject to a special density constraint, namely, that form-priming only occurs for words that have few orthographic neighbors and hence are located in low-density regions of the lexicon. However, it has been reported that no such constraint applies to the naming task, which shows strong priming for short words regardless of the neighborhood density. It is shown that performance in the naming task is subject to a special Stroop-like interference effect when the prime and target begin with different onsets. When this interference effect is held constant by matching onsets across conditions, the naming task exhibits the same properties as the lexical decision task. Further analysis shows that this onset effect is not obtained for all words. Words from low-density regions show no such effect, nor do low-frequency words or irregular words. These effects are interpreted in terms of response competition arising from a nonlexically mediated naming response to the prime. When a modified naming task is used (name-if-a-word), which requires the inhibition of all naming responses until lexical processing is completed, no onset effect is obtained.