This investigation measured effects of the modern mathematics on achievement in traditional arithmetic and in modern mathematics. Material on Sets, Variables, and Statistics was taught as a separate subject to an experimental group of 207 sixth-grade students. Instruction in the new mathematics content was given over a four-month period, using one-fifth of the regular arithmetic time each week. A matched control group of 193 students continued with the regular arithmetic program. The California Arithmetic Test was administered as a pretest and post-test to measure achievement in traditional arithmetic, while the Elementary Mathematics Concepts Test was administered as a post-test to measure achievement in modern mathematics. The procedure was repeated in 1966 with an experimental and control group of 180 and 206 respectively. The findings reveal that (1) the substitution of instruction in new mathematics topics for one of the regular arithmetic lessons each week does not affect student achievement in traditional arithmetic, (2) recent mathematics materials have had observable effects on student achievement in traditional arithmetic, and (3) the substitution of instruction in new mathematics topics for one of the regular arithmetic lessons each week does affect student achievement in modern mathematics. (RP)