We measured growth rates and RNA/DNA ratios of deposit-feeding bivalves Abra ovata cultured for five months in the laboratory where they were fed on fresh detritus derived from twelve macrophytes (Rissoella verruculosa, Cystoseira mediterranea, Corallina elongata, Stypocaulon scoparium, Padina pavonica, Ulva rigida, Cystoseira compressa, Colpomenia sinuosa, Posidonia oceanica, Codium vermilara, Asparagopsis armata, and Dilophus spiralis). Growth rates and RNA/DNA ratios were significantly affected by the nature of the detritus offered. However, there was no significant (n = 65, r = 0,07) correlation between growth rates and RNA/DNA ratios. These results show that there were significant qualitative differences in the tested detritus. They suggest that the use of RNA/DNA ratios would be inefficient in predicting growth rates of field-collected Abra ovata.