Purpose: To examine the physiological and perceptual responses during self-regulated exercise in children with cystic fibrosis (CF). Methods: Twelve children (10.8 ± 2.0 y) with CF made 3 separate visits to the research laboratory in the school of Health and Human Performance in DCU. The first visit was used to determine VO2peak and to anchor the children’s OMNI RPE scale. The second visit was used to familiarise the children with the testing procedures. During the final visit children exercised for 20 min on a cycle ergometer. The exercise intensity was self-regulated. Heart rate (HR) and VO2 were measured using telemetry and open circuit spirometry, respectively. Undifferentiated RPE (RPE-O) and differentiated RPE for the chest (RPC-C) and legs (RPE-L) were assessed using the Children’s OMNI RPE scale. Results: Perceptual and physiological responses remained stable after the first 5 min of exercise. Subjects exercised at 53% VO2peak and 73% HRpeak during the final 15 min of self-regulated exercise. RPE-O was 5.8 ± 2.6, RPE-C was 4.5 ± 2.6, and RPE L was 5.7 ± 2.4. The %VO2peak and %HRpeak corresponding to the ventilatory breakpoint (Vpt) was 63% and 82% respectively. Two thirds of the subjects selected an exercise intensity ±20% VO2@Vpt, and one third selected an intensity 31 - 38% below VO2@Vpt. Conclusion: Although the mean %VO2peak was within the moderate to vigorous intensity range recommended for health and fitness benefits, there were large intra individual variations in RPE, %VO2peak, %HRpeak and %Vpt during self regulated exercise in children with CF.