Citric acid production from mussel processing effluents by Aspergillus niger in solid-state culture was studied using polyurethane foam particles soaked with the culture medium. Conditions were used that allowed comparison of the results with those obtained before in submerged culture and the attribution of the differences to the characteristics of solid-state culture.A screening of several strains gave different results than in submerged culture and a reduction in the incubation time. The joint effect of nitrogen and phosphorous initial concentrations was examined by means of orthogonal factorial designs in five selected strains. The results showed that different requirements of N and P for each individual strain were needed. Optimization was done using different experimental strategies for two selected strains. Following either the direction of the linear approximation gradient of the first-order empirical model obtained or with a new rotatable factorial design, an optimum value could be obtained.In comparison with previous results in submerged culture, the strains,vith high requirements of N and P seemed to be disfavored in solid-state culture. The ones with low requirements were favored and showed a bigger tolerance to a surplus of both nutrients.