ABSTRACT: Larval settlement in the serpulid polychaete Hydroides elegans (Haswell) reportedly occurs in response to both an attractive bacterial film and polar, aliphatic dissolved free amino acids (DFAA). In this study, we tested whetherthe larval metamorphic response was caused by direct DFAA perception or by induction of DFAA-utilizing bacteria in assay dishes. Still-water laboratory assays were performed with sterile measures in the presence and absence of an antibiotic<c><c>mixture (2.5 x 10^-4 M streptomycin, 1.0 x 10^-4 M penicillin). In the absence of these antibiotics our results revealed a significant correlation between DFAA decrease rates, bacterial film development and larvalmetamorphosis within a 24 h assay period. After 12 h, DFAA concentrations decreased below the detection limit of 0.013 µM, the attached bacterial density in assay dishes was approx. 2 x 10⁴ cells mm^-2, and larval metamorphosis wasinsignificantly low (20%). After 24 h, DFAA were no longer detectable, the attached bacterial density was unchanged, and larval metamorphosis was significantly high (50%). In contrast, naturally biofilmed dishes showed a significant metamorphic responseof 50% after 12 h. In analogous experiments in the presence of antibiotics, the percentage of larval metamorphosis varied between 3 and 34% after 24 h while the number of attached bacteria was low (0.5 to 5 x 10³ cells mm^-2). Weassume that the metamorphic response in these treatments was caused by inductive, antibiotic-resistant bacteria. Our results suggest that larval metamorphosis was exclusively triggered by an inductive bacterial film rather than by direct larval perceptionof DFAA<i>.</i> Despite most sterile measures at the set-up of bioassays, bacteria were inevitably inoculated into test dishes as they were found associated with the larva and suspended in the larval culture water. Therefore, our results point to asystematic error in this established assay procedure. If putative signaling compounds serve as a nutrition source for larval settlement inducing marine bacteria, we conclude that an explicit investigation of a chemical metamorphic cue's efficacy isunreliable. The ecological significance of the transformation of DFAA into an attractive bacterial film for larval attachment and metamorphosis in <i>H</i>. <i>elegans</i> is discussed.</c></c>