Kenya re-implemented Free Primary Education (FPE) in January, 2003 leading to an increase in student enrolment in some classes more than in others. This could have had far reaching implications on the quality of education of children. Consequently, this study aimed at establishing the influence of increased student enrolment on the techniques used to teach science in upper primary schools. The study employed descriptive survey research design. The sample size was 108 questionnaire respondents and 36 interview respondents, respectively. However, the study only managed to achieve 116 (90 questionnaire and 26 interview) respondents. Sampling involved multi-stage sampling, purposive sampling, proportionate purposive sampling and simple random sampling. Data collection was done using Questionnaire for Science Teachers (QST) and Science Teachers’ Interview Schedule (STIS). The data collected were analyzed using Statistical Package for Social Sciences (SPSS). The significance level of the differences between mean frequencies of use of the teaching techniques was done at the a value of 0.05. The study found that increase in student enrolment influenced use of lecture technique positively while the use of practical, project and assignment techniques were influenced negatively. Such enrolment did not influence demonstration, discussion, field course excursions and question and answer techniques. Consequently, increase in student enrolment impacted negatively on the acquisition of science process skills and attitudes.