The purpose of this research was to study the Lexico-semantic errors of the Keiyo-speaking standard seven primary school learners of English as a Second Language (ESL) in Keiyo District, Kenya. This study was guided by two related theories: Error Analysis Theory/Approach by Corder (1971) which approaches L2 learning through a detailed analysis of the learners' own language and Interlanguage Theory by Selinker (1972) which looks at the language of L2 learners as an interim grammar advancing towards TL. The two theories complemented each other for the identifications, analysis and categorization of the learners' errors as found in the composition, story telling and teachers questionnaires. The data was classified, analyzed and interpreted through descriptive and inferential statistical technique to test the hypotheses. Data was presented in form of tables and figures. From the learners' compositions and story telling tape recorded, two types of broad error categories were established: Interlingual and Intralingual. Interlingual errors which comprised 35.74% of the total number of errors made, included phonologically induced, language switch, paraphrase, calques and epenthesis. The intralingual errors were the majority. They comprised 64.27%. These errors included: semantic contiguity, coinage, malapropisms, learning induced, collocation and ignorance errors. From the data, though L1 errors were few, it hampers both communication and affects the affective component of the learners. In the findings of this study the nature and causes of the errors found can be used to get solutions to learners' poor performance in English language.