Influence of temperature on development and mortality of immature stages ofRhinocyllus conicusFroelich was investigated under different constant temperatures in the laboratory. Development time functions based on the parabola were used to model the data. Because the larvae develop within musk thistle (Carduus thoermeriWeinmann) heads, developmental times of the larvae were measured indirectly. Determination of the four larval stages ofR. conicuswas based on distribution of head-capsule sizes of field-collected larvae. Hatching threshold was 11°C and threshold temperatures for development were 16–20°C for the different larval and pupal stages. This temperature range may be an adaptation ofR. conicusfor synchronization with musk thistle. Highest mortalities for immature stages ofR. conicusoccurred in the first and fourth instars.