This research analyzes the skills of reading words and pseudo-words, reading speed and reading aloud fluency in Chilean children of 3rd (n=41) and 4th (n=40) year of primary education in three public schools from Concepción. We analyzed objective measures of accuracy, speed and word and pseudo-words reading efficiency; subjective measures of reading fluency resulting from the application of a scale to evaluate the reading aloud of an appropriate text to each school level, and the objective measure of reading speed. Significant differences were found in favor of 4th graders in the speed and efficiency of reading words and pseudo-words, and in objective reading speed. In reading fluency, we just found differences in the handling of pauses. Nevertheless, in both groups there were significant correlations between word and pseudo-word reading and fluency. However, while in 3rd grade the efficiency of word reading is the variable that can most explain the performance in fluency, in 4th grade it is objective reading speed. The results also indicate that the efficiency of reading pseudo-words is more decisive in third grade than in fourth grade. It is concluded that a greater development of the automation of word reading favors the development of a greater reading speed and with it a greater fluency, since intrusive hesitations and pauses are reduced.En este estudio se describe la fluidez lectora (precisión, velocidad y prosodia) en escolares chilenos de 3° y 4º año básico a través de una escala subjetiva y sus relaciones con la velocidad lectora objetiva y la eficiencia del uso de las rutas directa e indirecta de la lectura de palabras. Los resultados indican que en 3° año básico la eficiencia de la ruta directa es la variable que más explica la fluidez, mientras que en 4° año básico lo es la velocidad objetiva. Se concluye que la velocidad permite diferenciar el desarrollo de la fluidez en estos niveles escolares.