In sentence processing, retrieval cue parsing accounts predict that processing difficulty occurs due to interference between similar noun phrases at verb integration (Van Dyke & Lewis, 2003; Van Dyke, 2007). An important factor is number marking of the nouns because the number of the verb and the subject has to agree. According to Lewis and Vasishth (2005), different types of retrieval cue overlap (e.g., verb subcategorisation, semantics, number) should all cause interference simultaneously during processing. However, Van Dyke (2007) reported that interference due to subcategorisation overlap preceded semantic interference, suggesting that syntactic interference may occur before interference due to other cues. We investigated whether interference due to number overlap also occurs later than interference due to subcategorisation overlap. Interestingly, using eye-tracking, we found that the number congruency effect occurred early and no later than the subcategorisation interference effect, indicating that number interference has a very rapid effect on sentence processing.