The in situ enzymatic polymerization of aniline onto multi-walled carbon nanotubes (MWCNT) and carboxylated MWCNT (COOH-MWCNT) is reported. Nanostructured composites were prepared by this method. Polymerization was catalyzed with the enzyme horseradish peroxidase at room temperature in aqueous medium of pH?4. Hydrogen peroxide was used in low concentration as the oxidant. The nanocomposites were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The TEM studies showed tubular morphology with uniformly distributed MWCNT in the nanocomposites. The SEM and TEM investigations revealed wrapping of the MWCNT with polyaniline (PANI) chains. TGA demonstrated that the PANI component is thermally more stable in PANI/COOH-MWCNT compared to the PANI/MWCNT composites. The synthesized nanocomposites showed higher conductivity than pure PANI, which may be due to the strong interaction between the PANI chains and the MWCNT.