This study was conducted to determine if fifth-grade students can significantly improve their use of logical analysis through a suitable instructional unit taught under ordinary classroom conditions. Concrete teaching materials were developed to familiarize students with the distinction between the valid inference patters--Modus Ponendo Ponens and Modus Tollendo Tollens (MP, MT), and the fallacious ones--Affirming the Consequent and Denying the Antecedent (AC, DA). No formal rules were taught. The experimental unit was implemented 4 to 5 times a week for 23-25 sessions, by 4 fifth-grade teachers in their ordinary classes. The teachers participated in a 12-hour pretraining workshop. A pretest/posttest, treatment/no treatment design was applied to assess resulting improvement in students' conditional reasoning ability. The sample consisted of 210 fifth-grade students in a suburban area, 104 in 4 experimental classes and 106 in 4 control classes. Experimental and control group pretest performance levels did not differ, but there was a significant difference in the posttest means. There was no significant change in the control group's pretest and posttest performance levels on any logical form, or for the experimental group's on MP and MT. However, on AC and DA the two groups' gain scores were significantly different. (MLH)