Small samples of two wood-polyethylene (HDPE) composite formulations were incubated with either the white-rot fungus Trametes versicolor or the brown-rot fungus Gloeophyllum trabeum for 24 and 77 days in an agar-block test. Noninoculated, side-matched controls were employed in the tests to serve as references, and solid wood samples of yellow-poplar (Liriodendron tidipifera L.) inoculated with T. versicolor were included as positive controls. Potential changes in storage and loss moduli because of fungal colonization and moisture were determined using dynamic mechanical analysis, whereas weight loss and visual observation served as indicators of fungal decay. Severe losses in storage modulus (E') and loss modulus (0) following incubation of yellow-poplar with T. versicolor were observed. However, the E' of the two wood-plastic composite (WPC) formulations increased after 24 days of incubation with T. versicolor. The same effect was observed for G. trabeum, but only in one formulation. The increase of E' was attributed to a reinforcing effect of the fungal hyphae present in the interfacial gaps between the wood filler and the polymer matrix. Dynamic temperature scans revealed a peak in E" between 30 degrees C and 63 degrees C, depending on the frequency and fungal treatment. The peak temperature of E" represents the a-transition of HDPE. Increased activation energies were required for the a-transition in WPC samples incubated with T. versicolor for 77 days as compared to controls. This observation confirmed that incubation of WPC with T. versicolor improved interfacial adhesion and reinforced the composite under the assay conditions. (c) 2006 Wiley Periodicals, Inc.