The article presents results of biogas generation from sewage sludge after applying two pretreatment methods: sonification and thermal hydrolysis. Original results and literature data of the ultrasonic field influence on biogas generation were compared with literature data concerning effectiveness of the thermal hydrolysis. Sludge pretreatment by the ultrasonic field intensified the biogas production as the amounts of biogas was of ca. 20-24% higher, as compared to the nontreated sludge. The highest generation of biogas was observed after a shorter time. The degree of organic matter reduction was of 45-47%. The content of volatile fatty acids dropped down to 139 mg CH3COOH dm(-3) during 20 days. During the thermal pretreatment of sludge the amounts of generated biogas were ca. 25% higher, as compared to nontreated sludge. The maximum biogas production of 0.92 dm3 was observed in the day 9 of fermentation. To-date results indicate that efficiency of ultrasonic disintegration depends on sonification time, type of heads, as well as power and frequency transmitted to heads.