The efficiencies of free and immobilized bacterial cultures of petroleum hydrocarbon degraders were evaluated and compared in this study.Hydrocarbon-degrading microbial communities with high tolerance to and high degrading ability of crude oil were obtained from the soil contaminated with crude oil in the Yellow River Delta.Then,the microbial ceils were immobilized in sodium alginate (SA) beads and sodium alginate-diatomite (SAD) beads.The biodegradation of crude oil in soil by immobilized cells was compared with that by free cells at three inocuIation concentrations,1 × 104 colony forming units (cfu) kg-1 (low concentration,L),5 × 104 cfu kg-1 (medium concentration,M),and 1 × 105 cfu kg -1 (high concentration,H).At 20 d after inoculation,the maximum degradation rate in the immobilized systems reached 29.8％ (SAD-M),significantly higher (P ＜ 0.05) than that of the free cells (21.1％),and the SAD beads showed greater degradation than the SA beads.Moreover,both microbial populations and total microbial activity reached significantly higher level (P ＜ 0.05) in the immobilized systems than free cell systems at a same initial inoculation amount.The scanning electronic microscope (SEM) images also confirmed the advantages of the immobilized microstructure of SAD beads.The enhanced degradation and bacterial growth in the SAD beads indicated the high potential of SAD beads as an effective option for bioremediation of crude oil-contaminated soils in the Yellow River Delta.