The use of radiolabelling and high performance liquid chromatography (h. p. l. c. ) has shown that the freshwater pulmonate snail
Biomphalaria glabrata
can achieve a net uptake of short chain carboxylic acid (C
2
-C
4
) from the medium, and also metabolize them. This appears to be the first time that this phenomenon has been reported for a freshwater invertebrate. The uptake characteristics for acetate and butanoate conform to the Michaelis-Menten model. The transport constants,V
max
(in micromole equivalents per gram of wet mass per hour) and K
s
(in micromoles per litre) for acetate (1005 and 2.29 respectively) and butanoate (129 and 1.29 respectively) are similar to those obtained by other workers for marine, detritivorous, polychaete worms. Evidence is given that the carboxylic acids are transported through the body wall, and not taken up to any significant extent by microorganisms or absorbed on the surface mucus. The acids are metabolized by the snail after accumulation, as shown by the appearance of label in respiratory CO
2
and the decline in the mass-specific accumulation rates (m. s. a. r. ), in micromole equivalents per gram of mass per hour, to an asymptote during the course of a 4 h incubation period. Differences in m.s.a.r. values for [1-
14
C]- and [2-
14
C]acetate probably reflect the different fates of the two carbons during metabolism. The m. s. a. r. values are influenced by the metabolic state of the snails. Thus they increase in response to food deprivation and decrease under anoxic conditions. However, as snails continue to accumulate acetate from the medium, even when well fed, or ‘preloaded ’ with acetate, it is clear that this acid is not being used solely as a food supplement. Estimates of the contributions that C
2
and C
4
acids might make to the basal metabolism of the snails have been made using the following param eters: (i) the range of concentrations of these acids in microhabitats, such as the interstitial waters in surface sediments, and an experimentally decayed macrophyte,
Lemna paucicostata
(ii) transport constants of the carboxylic acids taken up by the snails; (iii) the respiration rates of the snails. It was calculated that the acetate in interstitial waters can contribute only a negligible amount (less than 6%) of the snail’s basal metabolic rate (b. m. r. ). However, if the snails encountered the concentrations of 2250 μm and 400 μm of acetate and butanoate respectively found in supernatant from laboratory decaying
Lemna
, then each acid could provide more than 50% of the snails’ b. m. r. The significance of these results to the behavioural and chemical ecology of the snails is discussed.
