On the basis of previously published experimental data, the catalytic cycle of Escherichia coli galactosidase has been reconstructed. The enzyme plays a key role in the transcription regulation of lac operon. Using the proposed scheme, we have built a kinetic model that takes into account the reactions of hydrolysis, isomerization as well as galactosylation. Using the experimental data and optimization algorithms, all parameters of the model have been estimated. On the basis of the model, the features of beta-galactosidase functioning have been predicted in a wide range of conditions.