This study investigated the comparative efficiency of Web-based instruction (WBI) and traditional teaching methods on preservice teachers' fraction knowledge. Students' knowledge of fractions was measured using a Fraction Knowledge Test. The test consisted of 32 items and was administered as pre- and posttests to a total of 42 preservice teachers in two intact classes at the same university. One of the classes was randomly assigned as the experimental group (n = 21) and was given WBI. The other class was assigned as a control group (n = 21) and was given traditional instruction. Analysis of covariance was used to determine treatment effects on students' knowledge of fractions when the pretest result was used as a covariate. The analysis of results showed a statistically significant difference between the experimental and the control groups' posttest mean scores in favor of the experimental group. (Contains 7 tables and 3 figures.)