ABSTRACT: Several problems which can occur in the evaluation of the coefficient of transmissivity from pumping tests on fractured rocks are discussed in this paper. Attention in the first part is focused on possible effects on ground water level fluctuations during the pumping test. These effects reflect the properties of rocks and therefore influence the parameters calculated from the pumping test results. In this case, the calculated parameters do not represent a real aquifer. The second part of this paper outlines the potential impact of the presence of several aquifers on estimation of hydraulic parameters. In the third part, some problems in the coefficient of transmissivity estimation are discussed, using the linear approximation method of Jacob, and these are illustrated by examples of wells existing in a neo-volcanic rock environment.