Different studies show pupils misconceptions about some of the biological subjects. Our objective was to examine pupil’s conceptions about marine organisms. We were interested in the difference about conceptions based on sex, on 7th and 9th graders and on the difference between pupils who explore marine organisms on their vacations and those who don’t. We wanted to test which organisms’ characteristics help pupils classify living beings into higher systematic classification. For our study we used an online survey and the method of the measurement of the reaction time. We wanted to check the usefulness of relation between both methods for the examination of conceptions about our topic. In the part of the study in which the reaction time was measured, participated 65 7th and 9th graders, and in the survey 19 more pupils of the same grades joined. With the help of the results we concluded, that differences in the knowledge about marine organisms, based on sex, don’t exist. The differences were also negligible in the comparison between different grades and between pupils that do and those who don’t explore marine organisms while on vacation. In the majority of pupils we noticed the presence of some of typical misconceptions about our topic. However, the measurement of the reaction time didn’t show us those same results that online survey did. By comparison of classification marine organisms among plants or animals in both methods we claimed, that both methods had showed presence of the same conceptions about marine organisms.