The study examined the effect of birth order differences on adjustment among first year undergraduate students in one selected university in Kenya. A cross-sectional survey research design was adopted. The sample size comprised 213 first year students selected using both stratified and simple random sampling techniques. The adjustment questionnaire was used to collect data. The inferential statistics such as Analysis of Variance (ANOVA) and Tukey's post hoc tests were used to analyze data. The ANOVA results indicate a significant effect, [F (5, 207)=8.279, p=0.000], of order of birth of first year students on their level of overall adjustment. The Tukey's HSD Post Hoc test results indicate that 1st born students presented a significantly higher overall adjustment compared to the rest. The study recommends that staff at university counseling centers should develop specific orientation programmes to enhance the adjustment of first year students who occupy later orders of births in their families apart from first born students.