Abstract: Two sequenced screening trials were carried out on 21 fungi isolated from decayed rice straw. The aim was to obtain fungal isolates with cellulolytic activity and rock phosphate (RP) solubilization ability for employing them in organic acid production from rice or wheat straw. Penicillium, Aspergillus and Stachybotrys were the predominant isolated genera. On the base of cellulase (filter paperase, carboxymethyl cellulase and β-glucosidase) production, RP solubilization efficiency and antagonism tests, two fungi were selected and molecularly identified as A. niger GU 295947 and P. chrysogenum GU 295948. Under solid-state fermentation (SSF) conditions of rice straw with RP, the maximum cellulase production and RP solubilization were recorded after 4 weeks incubation in the presence of 75 mg P2O5 from RP and 7.5% (v/w) fungal inoculum. Applying these fermentation conditions on rice and wheat straw, using individual or dual inoculation of both fungi led to more than 40% loss in the weight of fermented straw, this was accompanied with the releasing of glucose and soluble phosphorus in the hydrolysate of fermented straw. Using high-performance liquid chromatography, the resulted organic acids by A. niger GU 295947 and/or P. chrysogenum GU 295948 in the hydrolysate of SSF were investigated. Acetic, ascorbic, citric, formic, itaconic, levulinic, maleic, oxalic and succinic acids were detected. Their presence and concentrations were varying according to the substrate and the microorganism. The most noticeable thing was the broadspectrum varieties and the higher concentrations of produced organic acids when rice or wheat straw was used in combination with RP. Oxalic acid was the highest (40.0 mg g-1 straw) detected organic acid in the hydrolysate of fermented straw with RP by P. chrysogenum GU 295948. This study suggests production of organic acids especially, oxalic and succinic acids as the major organic molecules by such fermentation.