Due to ever decreasing response and return rates, finding a cost-effective method for following up media research panels is an increasingly important issue, especially for radio listening research (Gendall and Davis 1993). This paper reports the findings of an experimental study that examined the effectiveness of two different modes for following up members of a radio diary panel. The modes tested were telephone contact and a mailed letter. The combined follow-up strategies increased the return rate of completed diaries by 23 percent. However, there was no real difference in either the return rates or the cost effectiveness of the two modes. The telephone treatment eliciting just three more returned diaries than the mail treatment, and cost $1.17 per return compared with $1.22 for the mail treatment. These results suggest that, for improving the return rate in radio research, it makes no difference whether non-respondents are followed up by mail or telephone.