The purpose of this paper is to report the results of an investigation of the interrelationships among extensive reading, word-guessing strategies, and incidental vocabulary acquisition. The participants of this study were two English classes for college freshmen taught by the researcher at a university in Taiwan during the school year of 2002 (fall 2002-spring 2003). In both classes, the same English textbook was used for English-teaching purposes. In addition, they were required to read the same 12 graded readers for outside reading during the time of the study. However, one of the classes, called the ERWG group, was selected to receive training in class on how to guess the meaning of unknown words from context. The second class, called the ER group, was directly given explanations for the unknown words in the textbook without receiving any specific training on word-guessing strategies. The results of this study revealed: (1) the extensive reading program significantly increased the vocabulary size and reading proficiency of both groups; (2) the ERWG group made more significant improvement in word recognition than the ER group; (3) the word-guessing strategy training did not cause the ERWG group to make significantly more improvement in reading proficiency than the ER group; (4) neither of the two groups made significant improvement in their ability to guess the meaning of unknown words from context; (5) only the higher-proficiency readers in the ERWG group significantly improved their ability to guess meaning from context. The teaching implications of these findings are discussed at the end of the paper.