This study investigates whether intermediate Japanese learners of English (JLE) show any variability in sensitivity to the overuse of 3rd person singular (3sg) -s and if they do, what the causes may be. The results of the experiment indicate that JLE do exhibit variability: JLE showed sensitivity to ungrammaticality caused by a discrepancy in person features between subjects and verbs. In addition, they were sensitive to number feature disagreement when the plurality of subjects was expressed syntactically, namely, by using the conjunction and (e.g., Tim and Paul), by the demonstrative these and by a numeral quantifier (e.g., these two secretaries). However, they were not sensitive to such disagreement when subjects were marked only by plural -s (e.g., The chefs). Based on these results, it is suggested that the failure of JLE to use 3sg -s may not lie in the difficulty of subject–verb agreement, but in the detection of the number feature of sentential subjects. Intermediate JLE appear to have problems both with the number feature at the lexicon/syntax level and with its morphological mapping at the level of morphology.