Plastic films containing cornstarch (40% dry weight) in combination with polyethylene (PE) and polyethylene-co-acrylic acid (EAA) were exposed to river and pond habitats for up to 60 days. Fourier transform infrared (FTIR) spectroscopic analysis of the starch-plastic films indicated that up to about 40% of starch in the films disappeared after 60 days' exposure. PE and EAA components were unchanged during this period. A diverse microbial biofilm quickly accumulated on each of the plastic films in both environments. Although the amount of biofilm accumulation was greater in the river environment than in the pond environment, there was little quantitative difference in the extent of biofilm formation on plastic films composed of PE, EAA, or starch-PE-EAA in either environment. All three types of plastic films were heavily colonized by a midge-fly larva (Dicrotendipes sp.) by 60 days of exposure in the aquatic environments, but only starch-containing plastic films showed any evidence of physical damage that could have been a result of ingestion of the plastic by the larvae (pond > river), suggesting that, in addition to microbial starch degradation, biodisintegration may also be a significant factor influencing the environmental fate of starch-containing plastics.