Misconceptions are a special case of false beliefs. They should be both robust and important to a person's belief system. Chi (1992) has asserted that in some domains, such as the circulatory system, students initial conceptions are of the same general ontological class as the textbook conceptions. They should therefore not be as robust as the initial conceptions in domains such as physics, in which the initial conception may be of the wrong class. The initial beliefs of 12 eighth grade students about the circulatory system included a variety of false beliefs. Statements of both correct and incorrect beliefs were used to generate maps of students' initial mental models. This allowed an assessment of the importance of the false beliefs. Even deeply embedded beliefs were removed by instruction. Importance was also measured by the impact of false beliefs on a pre-test, which was not significant. Resistance to instruction was tested by having students read a text. One analysis checked individual false beliefs, to see if contradiction by the text resulted in false belief removal. Beliefs which were contradicted were generally removed. These results are not consistent with the notion that students bring with them to instruction important and robust misconceptions about the circulatory system.