An electrochemical biosensor was constructed based on the immobilization of myoglobin (Mb) in a composite film of Nation and hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) for a modified carbon paste electrode (CPE) Direct electrochemistry of Mb in the Nation-BMIMPF6/CPE was achieved, confirmed by the appearance of a pair of well-defined redox peaks The results indicate that Nation-BMIMPF6 composite film provided a suitable microenvironment to realize direct electron transfer between Mb and the electrode The cathodic and anodic peak potentials were located at -0.351 V and -0.263 V (vs SCE), with the apparent formal potential (Ep) of -0.307 V, which was characteristic of Mb Fe(III)/Fe(II) redox couples. The electrochemical behavior of Mb in the composite film was a surface-controlled quasi-reversible electrode process with one electron transfer and one proton transportation when the scan rate was smaller than 200mV/s Mb-modified electrode showed excellent electrocatalytic activity towards the reduction of trichloroacetic acid (TCA) in a linear concentration range from 2.0 x 10(-4) mol/L to 1.1 x 10(-2) mol/L and with a detection limit of 1 6 x 10(-5) mol/L(3 sigma) The proposed method would be valuable for the construction of a third-generation biosensor with cheap reagents and a simple procedure (C) 2009 Elsevier Ltd All rights reserved.