Bubble collapse times were determined as a means to characterize wall velocity vs. drive field in thin (3 to 11 μm) epitaxial garnet films of compositions near Y3Ga1Fe4O12, Eu0.5Y2.5Ga1Fe4O12 and Gd0.5Y2.5Ga1Fe4O12. The results resemble that reported for thick platelets ( > 25 μ thick) with large diameter bubbles ( d ≈ 25 μ). An initial steep slope (∼103 cm/sec‐oe) is followed by a curve of diminishing slope. The deviation from linearity occurs well below Walker's limit of velocity Vw = 2πγMs▵valid for free wall motion in a plate of infinite thickness. Our theory shows that when damping (Gilbert's a) is small, stray fields near the film surface destabilize the linear uniform motion. At fields above the limit Hc = 9. 5 a x (2πA)1/2h−1 irregular Bloch line motion occurs and the mean wall velocity approaches the asymptote V o = 7.1 γ Ah −1 K 1/2 To the extent that experimental knee velocities are well defined, they range from 1/2 to 1 times Vo.