Phosphor-us containing fertilizers have an important role in agriculture. Conventionally phosphate fertilizers are obtained by rock phosphate (RP) dissolution using mineral acids. Biotechnological methods can be a promising alternative in RP processing. The influence of Aspergillus niger strain, the composition of a nutritive medium, Morocco phosphorite (MP) concentration in the liquid medium, the time of bioconversion and the preliminary mechanical activation (PMA) of MP on the phosphorite microbial solubilization has been presented. The phosphor-us concentration (as P2O5), citric acid production, glucose concentration and pH in the cultural medium were monitored. The phosphate concentration was expressed as water soluble - alpha(1) (in the native cultural liquid), citrate soluble - alpha(2) (after treating the biomass and remaining mineral mass with citric acid) and biomass available phosphorus - alpha(3). Phosphate dissolution was not strongly correlated both with the citric acid production and the incubation period. When the fungi were grown without water soluble phosphor-us compounds the MP solubilization had higher values. A maximum of 94.80% total P2O5 extraction was achieved. The PMA activity does not facilitate MP dissolution during the bioconversion. (C) 2007 Elsevier Ltd. All rights reserved.