A diverse diet including animal source foods (ASF) is essential for pregnant and lactating women, as well as infants and young children (IYC) 6-23 mo to promote healthy growth.In rural impoverished areas of Tanzania, where stunting prevalence is high and dietary diversity low, the Enhancing Nutrition Services to Improve Maternal and Child Health (ENRICH) project aims to improve the health and nutrition of women and IYC.To identify nutritious priority foods and behaviors to promote in this context, formative research was conducted.Our objective was to identify priority ASF to promote increased dietary diversity and improved maternal and IYC nutrition within the local context.Using a social-ecol. framework and an ethnog. approach, we employed qual. methods to collect data from purposive samples on potential key foods and behaviors.Food attributes, market surveys, household food stock, in-depth interviews, 24-h dietary recalls and home observations were conducted.Anal. of each method and triangulation of results led to selected ASF foods and behaviors, which were tested in household trials for final recommendations.Among the various ASF explored, eggs and small fish emerged with the greatest potential for increased consumption.Many pos. attributes of eggs were voiced, but some neg. attributes, primarily related to traditional cultural views, were also expressed.Small fish were associated with mostly pos. attributes.Compared with other ASF, eggs and small fish were commonly available and considered inexpensive.Interviews with pregnant women and parents of IYC demonstrated links among nutritious foods, healthy growth and future aspirations, yet dietary recalls showed poor dietary quality and low ASF intake.Results from household trials found eggs available, easy to prepare and acceptable by women and IYC.Similar results were found for small fish.Formative research provided context rich data from multiple perspectives to select priority ASF.Household trials provided further evidence of practical feasibility and acceptability, as well as improved understanding of how to promote egg and small fish consumption.This research informed the development of messaging and a behavior change strategy for this setting.