Internet panels are increasingly used for stated preference research. Because members of such panels receive compensation for each completed survey, one concern is that over time this creates professional respondents who answer surveys solely for the monetary compensation. We identify professional respondents using data on panel tenure, survey response frequency, completion rate and total number of completed surveys. We find evidence of two types of professional respondents: “hyperactives” who answer surveys frequently and “experienced” who have long panel tenure and a large number of completed surveys. Using an integrated choice and latent variable model on stated preference survey data, we find that “hyperactive” respondents are less likely to choose the 'status quo’ and have a more stochastic choice process as seen from the econometrician's point of view, whereas “experienced” respondents have a relatively more deterministic choice process. Our results show that “hyperactive” respondents significantly impact estimated values.