The low availability of phosphorus (P) in the soil and the high cost of P fertilization are factors that limit agricultural productivity. A biotechnological alternative for to handle this problem is to use soil microorganisms capable of dissolving rock phosphate (RP), thus improving its effectiveness as a P fertilizer. This study was carried out with the objective of determining the effectiveness of Aspergillus niger –As-, Penicillium sp. –Pn-, Bacillus sp –B-. and an unidentified actinomycete –At- in the in vitro dissolution of two partially acidulated rock phosphates. The treatments consisted of 2x16 factorial arrangement [2 levels of RP: either Boyaca RP or Norte de Santander RP; 16 levels of inoculum: an uninoculated control, individual inoculations (with As, Pn, B, At), dual inoculations (AsPn, AsB, AsAt, PnB, PnAt, BAt), triple inoculations (AsPnB, AsPnAt, AsBAt, PnBAt), and quadruple inoculation (AsPnBAt)]. Each treatment was replicated three times. Each treatment was replicated three times. It was found that the microbial effectiveness in the in vitro dissolution of RP depended on the type of RP, the composition of the inoculum used and the interaction of both factors. The best results were obtained with the Norte de Santander RP and A. niger used alone. When this fungus combined with the other microorganisms, its capacity to dissolve RP was significantly reduced. RESUMEN La baja disponibilidad de fósforo (P) en el suelo y el costo de la fertilización fosfórica son limitantes para la productividad agrícola. Una alternativa biotecnológica para manejar este problema es mediante el uso de microorganismos del suelo capaces de disolver rocas fosfóricas (RP) y así mejorar su efectividad como fertilizante fosfórico. Con este fin se realizó un ensayo para determinar la efectividad microbial en la disolución in vitro de dos RP (Norte de Santander y Boyacá) parcialmente aciduladas. Los tratamientos consistieron en un arreglo factorial 2x16 [2 niveles de RP: Boyacá o Norte de Santander; 16 niveles de inóculo: Un control no inoculado, inóculos individuales (Aspergillus niger –As-, Penicillium sp. –Pn-, Bacillus sp. –B-, y un actinomiceto no identificado –At-), inóculos dobles (AsPn, AsB, AsAt, PnB, PnAt, BAt), inóculos triples (AsPnB, AsPnAt, AsBAt, PnBAt), e inóculos cuadruples (AsPnBAt)]. Cada tratamiento tuvo tres replicas. La efectividad en la disolución in vitro de RP fue dependiente del tipo de RP, tipo de inóculo y la interacción de ambos factores, teniendo mejores resultados con la RP del Norte de Santander y A. niger sólo. Cuando este hongo se combinó con otros microorganismos su capacidad para disolver RP se redujo significativamente.