The claim that the syllable constitutes a basic perceptual unit in French is commonly accepted. It is based in part on the syllable effect [1] obtained with words. The present study extends these syllable detection experiments to pseudowords. Four experiments failed to replicate the syllable effect observed on words. Detection responses in pseudowords are made as soon as sufficient information becomes available in the signal. The different pattern of results obtained with words and pseudowords suggests that the syllable effect is post- lexical rather than pre-lexical.