The effect of thymine deprivation on the integrity of phage X, sex factor, and chromosomal deoxyribonucleic acid (DNA) in Escherichia coli CR34 (X ind-) was examined by sedimenting cell lysates through alkaline sucrose gradients. Both sex factor and chromosomal DNAs showed evidence of being degraded during the starvation period. In contrast, no loss of closed circular X DNA was observed.