Drift and deformation of sea ice fields in the Bothnian Bay are studied theoretically and on the basis of field experiment data covering two separate weeks in March and April 1977. The wind factor n and deviation angle 0 in wind-driven ice drift in the absence of internal friction are shown to depend on three non-dimensional parameters which include the ratio of wind speed to ice mass as a relevant quantity. For high wind speeds n and 0 were observed to be 2.8 % and 16°, respectively. The response of sea ice to moderate and high wind speeds is rapid, an approximate steady state being achieved in less than half an hour. Observations on deformation were mainly made in a 5 km scale. Typical deformation rates were 3-4. 10-3 h and the principal axes values tended to have opposite signs. The fast ice boundary had no notable influence on deformation of the 5 km scale areas, these being at a distance of 40-50 km and 20-25 km from the boundary. Some correlation is found between the vorticities of ice drift and geostrophic wind. Angular velocity of the large central floe correlated rather well with the 5 km scale vorticity. Deformation in the 50 km scale was studied for one day; rates were about twice as large as in the 5 km scale. The geometry of the fast ice boundary strongly affected the 50 km scale deformation. During a 13-hour storm period ridge density was estimated to have increased by 1.2-1.8 km-1 the Bothnian Bay.