Jaeggi and her colleagues claimed that they were able to improve fluid intelligence by training working memory. Subjects who trained their working memory on a dual n-back task for a period of time showed significant improvements in working memory span tasks and fluid intelligence tests such as the Raven's Progressive Matrices and the Bochumer Matrices Test after training compared to those without training. The current study aimed to replicate and extend the original study in a well-controlled experiment that could explain the cause or causes of such transfer if indeed the case. There were a total of 93 participants who completed the study, and they were assigned to one of three groups--passive control group, active control group and experimental group. Half of the participants were assigned to the 8-day condition and the other half to the 20-day condition. All participants completed a battery of tests at pre- and post-tests that consisted of short timed tests, a complex working memory span and a matrix reasoning task. Although participants' performance on the training task improved, results from the current study did not suggest any significant improvement in the mental abilities tested, especially fluid intelligence and working memory capacity, after training for 8 days or 20 days. This does not support the notion that increasing one's working memory capacity by training and practice could transfer to improvement on fluid intelligence as asserted by Jaeggi and her colleagues. (Contains 5 tables and 3 figures.)