We investigated spelling errors in English and Spanish essays by Spanish-English dual language learners in Grades 1, 2, and 3 (N = 278; 51% female) enrolled in either English immersion or English-Spanish dual immersion programs. We examined what types of spelling errors students made, whether they made spelling errors that could be due to crosslinguistic influence, and whether errors were associated with instructional program, English learner status, and grade level. Compositions were transcribed and coded using the Systematic Analysis of Language Transcripts (SALT) software. Spelling errors were suggestive of crosslinguistic influence that was mostly unidirectional from one language to the other rather than bidirectional. Spelling errors were related to instructional program such that students in Spanish-English dual immersion made more English spelling errors in English compositions due to Spanish influence, and students in English immersion made more spelling errors in Spanish compositions due to English influence. Students in higher grades also made less English spelling errors in English compositions due to Spanish influence than students in lower grades. These findings suggest that dual language learners acquire spelling patterns in one language influenced by instruction and home language, which transfers to spelling in the other language. [This is the online version of an article published in "Reading and Writing."]