The relative effectiveness of several testing instruments in predicting the reading achievement of Puerto Rican first graders was investigated in this dissertation. One hundred seventy-four students were tested by the "Reading Readiness Test (Prueba de Apresto)," the "Collective Puerto Rican Test of Mental Capacity (Prueba Colectiva de Capacidad Mental)," and the "Socio-Economic Study II (Estudio Socioeconomico II)." In addition, a medical examination was conducted on all students. Reading achievement, as measured by the "Reading Test-Spanish (Prueba de Lectura en Espanol)," was found to be significantly related to reading readiness, mental capacity, and socioeconomic status, but not related to health. The "Collective Puerto Rican Test of Mental Capacity" was found to be the best predictor of reading achievement for males, females, and urban students. The 'Reading Readiness Test" proved most accurate for rural pupils, and for all groups a measure of socioeconomic status improved prediction of reading achievement significantly. (Author/KS)