Aeromonads can utilize a wide range of low molecular-weight compounds, including amino acids, carbohydrates and long-chain fatty acids at a concentration of a few micrograms per liter. Utilization of biopolymers such as gelatin, casein and amylose is slow at this concentration level. The concentration of substrates available for an A. hydrophila strain in drinking water was usually below 10 micrograms of C/l. The autochthonous bacteria utilized these substrates more rapidly than the aeromonads. The multiplication of aeromonads in drinking water during distribution is therefore explained by their growth on biomass components in the biofilm and in sediments in the pipes.