Seventh and eighth grade students identified as holding an incomplete fraction conception of decimals were tested on related fraction knowledge. Most of these students (78%) had a problem in coordinating the size of the parts and the number of parts in comparing fractions. These students underwent several instructional sessions. Half of them worked on fraction coordination and on mapping it to decimals, while the other half had more instruction in decimals. Treating the source of the problem in fractions was found to be more effective in improving decimal conception. The remaining students (22%), who had a problem in decimals but not in fractions, improved in decimal conception following mapping instruction that promoted transfer of fraction knowledge to decimals. (Contains 4 tables.) [For complete proceedings, see ED500860.]