We examined how the development of familism values from 5th to 10th grade relates to 12th-grade prosocial tendencies (after controlling for 10th-grade prosocial tendencies) using a stratified random sample of 749 Mexican American adolescents (M = 10.42 years of age at 5th grade; 48.9% girls) from 35 culturally and economically diverse neighborhoods. Most of the families (44.3%) were at or below $25,000 in annual income. A 2nd-order linear growth model represented adolescents' familism values at 5th grade (intercepts) and change in familism values from 5th to 10th grade (slopes), with the vast majority of slopes being negative. Higher intercepts predicted greater compliant and emotional prosocial tendencies, and higher (i.e., more positive or less negative) slopes predicted greater dire (female adolescents only) and public prosocial tendencies at 12th grade. The results underscore the important role of familism values in prosocial development among Mexican American adolescents. (PsycINFO Database Record (c) 2018 APA, all rights reserved).