The focus of this article is the development and optimisation of a simple, sensitive, selective biosensor for the detection of heavy metals, through inhibition of beta-galactosidase (beta-gal) from Aspergillus oryzae after its immobilisation on an electrochemical transducer by crosslinking with glutaraldehyde. The inhibition of beta-gal by Cd(II) and Cr (VI) was demonstrated by the decrease of beta-gal activity monitored by the conductometric signal. Under optimal conditions, the developed voltammetric and impedimetric biosensor, based on the specific interaction of metallic ions with beta-galactosidase that increases the electron transfer of the redox probe, presented a wide range of linearity. Detection limits of 3.12 x 10(-10) M (91.7 ng/L) were obtained, using both techniques, for Cr(VI). For Cd(II) detection limits were 2.85 x 10(-8) M (6.95 mu g/L) using EIS and 3.22 x 10(-11) M (7.61 ng/L) using square wave voltammetry (SWV).