The aim of this work was to study the possibility of monitoring citric acid production from mussel processing wastes (MPW) by Aspergillus niger in solid-state culture (SSC) on an inert support. This was conducted by measuring CO2 and O-2 concentration in exhaust gases, using an automatic sampler connected to a gas chromatograph and a data acquisition system. The procedure permitted information on the physiological state of the culture to be obtained. A relationship between citric acid accumulation and a decrease in CO2 production was found, allowing citric acid production to be followed in real-time. Moreover, respiratory activity (mu(r),) can be estimated and the effect of different variables on this parameter studied. Initial nitrogen concentration, a critical factor for achieving high production of citric acid from MPW in submerged culture, revealed no effect in SSC. This indicates a tolerance of SSC to higher concentrations of nitrogen constituting an advantage when using residual media with high levels of protein and variability in their composition.