Background: Forage production in tropical soils is primarily limited by nutrient deficiencies, especially nitrogen (N) and phosphorus (P). The use of phosphate rock by plants is limited by its low and slow P availability and microbial phosphate solubilization is the main mechanism for P bioavailability in the soil-root system. The objectives of this study were (i) select a nitrogen-fixing bacteria which could be used as a co-inoculant with the Penicillium rugulosum IR94MF1 phosphate-solubilizing fungus and (ii) evaluate under field conditions the effect of inoculation combined with phosphate rock (PR) application on yield and nutrient absorption of a Urochloa decumbens pasture which was previously established in a low-fertility, acidic soil. Methods: Various laboratory and greenhouse tests allowed for the selection of Enterobacter cloacae C17 as the co-inoculant bacteria with the IR94MF1 fungus. Later, under field conditions, a factorial, completely randomized block design was used to evaluate the inoculation with the IR94MF1 fungus, the IR94MF1+C17 coinoculation, and a non-inoculated control. Two levels of fertilization with PR treatment (0 kg/ha and 200 kg/ha P2O5) were applied to each. Results: During five consecutive harvests it was observed that the addition of biofertilizers significantly increased (p < 0.05) the herbage mass and N and P assimilation compared to the non-inoculated control. However, no statistically significant differences were observed for the PR application as P source. Conclusion: P. rugulosum IR94MF1 is capable of solubilizing and accumulating P from the phosphate rock, making it available for plants growing in acid soils with low N content. These inoculants represent a good option as biofertilizers for tropical grasses already established in acidic soils with low N content.