This investigation examined developmental aspects of 155 second-graders' intrinsic reading motivation and reading achievement at the beginning and end of a school year. Reading motivation remained at the same level through the school year, while reading achievement was significantly higher at the end of it than at the beginning. According to word reading skills, low reading achievers (n = 63) and typical readers (n = 92) evinced significant differences in reading motivation. The former scored lower in overall reading motivation and self-concept as a reader, as well as in reading achievement. Throughout the school year, low reading achievers showed a decline in overall motivation, self-concept as a reader and literacy outloud, and did not improve in reading comprehension achievement. Reading motivation in typical readers remained high and steady during the school year. Implications of the results are discussed in relation to previous studies. HighlightsWhat is already known about this topic• There is a relation between reading motivation and reading achievement in upper-elementary school graders. • Intrinsic reading motivation is one of the factors related to success or failure in reading.