The purpose of this study was to evaluate a form of writing instruction designed to teach second-graders to write compare and contrast informational texts. 73% of the participants (N = 45) were White, 22% were Hispanic, 2% were Asian, and 1% were Black, with 63% of the students eligible for free or reduced lunch. 48% of the students were female and 52% were male. Data collection included pre- and post-instruction writing samples that were scored using both a holistic and analytic writing rubric. A paired samples t-test revealed statistically significant differences (p = 0.000 for both the holistic and analytic rubric analyses) in student writing scores before and after instruction with a medium (d = 0.52) and large (d = 0.95) effect size reported. These findings suggest that the integrated science and literacy instructional approach supported the second-graders as they learned to write science informational texts that employed a compare and contrast text structure. Because the analytic rubric produced ordinal data, a Wilcoxon Signed Rank Test was used to explore any differences noted for the individual rubric elements. Results in these analyses indicated that there were statistically significant increases from pre- to post-instruction for the use of science vocabulary and definitions, for word count, and for the use of periods. There were no statistically significant differences in the use of signal words to indicate the compare/contrast text structure, in the use of capitals, or an introduction or conclusion.