We examined the influence of semantic transparency on morphological facilitation in English in three lexical decision experiments. Decision latencies to visual targets (e.g., CASUALNESS) were faster after semantically transparent (e.g., CASUALLY) than semantically opaque (e.g., CASUALTY) primes whether primes were auditory and presented immediately before onset of the target (Experiment 1a) or visual with an stimulus onset asynchrony (SOA) of 250 ms (Experiment 1b). Latencies did not differ at an SOA of 48 ms (Experiment 2) or with a forward mask at an SOA of 83 ms (Experiment 3). Generally, effects of semantic transparency among morphological relatives were evident at long but not at short SOAs with visual targets, regardless of prime modality. Moreover, the difference in facilitation after opaque and transparent primes was graded and increased with family size of the base morpheme.