Long exposure observations at the Canada-France-Hawaii Telescope of 3C66B in the I filter are presented. After subtraction of the galactic background, optical emission on the counterjet side is detected in 10 knots coincident with the radio counterjet. Their radio-to-optical spectral indices (0.5--0.6) are typical of synchrotron emission in extragalactic jets, so that these knots possibly are the optical counterparts of the radio counterjet. If this is confirmed, 3C66B would be the first double-sided extragalactic optical jet. The optical counterjet would also be brighter than what is predicted from the relativistic beaming interpretation of brightness asymmetry between the two jets. This would thus prove that the radiation properties are intrinsically different in the two jets. Alternatively, these knots could have nothing to do with the counterjet. But it seems that in this case, the optical counterjet would be fainter than expected from the relativistic beaming interpretation, favouring intrinsic asymmetry as well. In addition, two new optical components are found in the jet.