We have been investigating students' knowledge pertaining to continuous charge distributions in introductory calculus based physics courses at Cal Poly Pomona. An open-ended questionnaire was developed based on feedback from three faculty who often teach introductory calculus based electricity and magnetism courses to engineering and physics majors in order to probe common observed difficulties. A pilot study was conducted (N = 145) and, based upon the feedback we obtained from these results, a slightly revised version of the questionnaire with three additional questions was subsequently given to a second sample of students (N = 263) in the following quarter. Our data suggests that students struggle with concepts related to linear, surface, and volume charge distributions, as well as the mathematical formulation of infinitesimal charge representations of continuous charge distributions. We will share specific examples and discuss the implications that our findings have for more effective instruction.