Quantitative literacy is essential to biological literacy (and is one of the core concepts in "Vision and Change in Undergraduate Biology Education: A Call to Action"; AAAS 2009). Building quantitative literacy is a challenging endeavor for biology instructors. Integrating mathematical skills into biological investigations can help build quantitative literacy. In our plankton population laboratory sequence, students test hypotheses about the influence of abiotic factors on phytoplankton populations by sampling experimental and control flasks over multiple weeks. Students track and predict changes in planktonic populations by incorporating weekly sample estimates into population growth equations. We have refined the laboratory protocols on the basis of student commentary and instructor observations. Students have reviewed the lab positively, and approximately one-quarter of them reported building their math skills by participating in the lab.