Defects in dislocation-free silicon single crystals grown by floating-zone method in the atmosphere containing Ar-H2 or H2 were investigated by means of chemical etching and X-ray projection topography. It has been found that anomalous etch pits and defect density are closely related to the hydrogen content. (Change of the defect density with hydrogen content shows a maximum at approximately 10% hydrogen for ay-grown crystals. After annealing at high temperatures, the defect density in the inner part of the crystal is found to be more than that at the surface and also more than that in as-grown state when hydrogen content is greater than 25% in the atmosphere.) Defects in annealed crystals grown in the pure hydrogen atmosphere have been observed. The relationship between some defects showing in topographs and etch patterns was found. Effect of annealing on wafer with these defects was also observed. Finally, clustering and precipitating of hydrogen have been discussed. Dislocation loops formed at precipitate-matrix interface were described. The process of motion and interaction of the loops as well as the forming of multi-centre loops at high temperatures were also discussed briefly.