In Aeromonas hydrophila, the ahyI gene encodes a protein responsible for the synthesis of the quorum sensing signal N-butanoyl-l-homoserine lactone (C4-HSL). Inactivation of the ahyI gene on the A. hydrophila chromosome abolishes C4-HSL production. The exoprotease activity of A. hydrophila consists of both serine protease and metalloprotease activities; in theahyI-negative strain, both are substantially reduced but can be restored by the addition of exogenous C4-HSL. In contrast, mutation of the LuxR homolog AhyR results in the loss of both exoprotease activities, which cannot be restored by exogenous C4-HSL. Furthermore, a substantial reduction in the production of exoprotease by the ahyI⁺ parent strain is obtained by the addition of N-acylhomoserine lactone analogs that have acyl side chains of 10, 12, or 14 carbons. The inclusion ofN-(3-oxododecanoyl)-l-homoserine lactone orN-(3-oxotetradecanoyl)-l-homoserine lactone at 10 μM in overnight cultures of A. hydrophila abolishes exoprotease production in azocasein assays and reduces the activity of all the exoprotease species seen in zymograms.