Estimation of aquatic plant biomass is a crucial component of successful management of aquatic ecosystems. In shallow water habitats, horizontal acoustics could be a promising approach for assessment of plant biomass, but tools for the interpretation of the acoustic characteristics of plants are needed. We first attempted to study the biomass of submerged aquatic plants by horizontal echosounding, with the goal to describe a basic acoustic feature, S, (volume backscattering strength), and relate it to plant biomass. We set up experimental and field studies to describe three common freshwater species: water persicaria (Polygonum amphibium L.); Eurasian watermilfoil (Myriophyllum spicatum L.); and sago pondweed (Potamogeton pectinatus L.). We studied 96 plants (single plants, small and large patches), 64 in the experiment and 32 in a shallow turbid lake (Neusiedlersee, Austria), using a SIMRAD EK60 echosounder, with a circular composite transducer (nominal angle 6.8 degrees). For all species, we found a positive linear relationship between S, and dry biomass, describing 67 to 83% of the variability. The slope of the relationship differed statistically between species. The S, of species overlapped, irrespective of their biomass; therefore, it was not suitable for species identification. The physiological/morphological states of the plants probably influence S, which was higher for species in the lake versus those in the experiment. Although gas, more than the biomass, is presumed to cause the plants reflectivity, we suggest an explanation of the relationship between these two variables. This study was a first step toward developing background research to create an operational system for shallow water studies of aquatic plants.