Basic science process skills (SPSs) constitute a key dimension of the scientific competence that should be developed from an early age. Therefore, preservice elementary teachers (PETs) who are preparing to teach science should have a sufficient mastery about it. This study presents a questionnaire that allows gradual levels of performance in SPSs to be established regarding formulating questions and hypotheses, identifying variables, measurement, planning an inquiry, and interpreting data. The questionnaire was validated in three phases: 1) during its design, with the external collaboration of a panel of experts, 2) by piloting of a draft versions of the questionnaire on a small sample of PETs, and 3) by applying the questionnaire to a sample of 435 PETs. Data analysis indicated that these were acceptably reliable, and the validity of the questionnaire was also verified. They also revealed the PETs' deficient levels of SPSs. This was especially so regarding identifying suitable questions for school inquiry, understanding precision in measurement, and interpreting data in a table. There was further support for the validity of the questionnaire and the reliability of the outcomes in that it corroborated other results in the literature on the subject that were obtained with different tools and methods.