The experiment was conducted in the plant protection laboratory of the Humera Agricultural Research Center to study the biology of sesame seed bug, Elasmolomus sordidus (Fabricius), Hemiptera: Lygaeidae. E.sordidus is considered as the most important harvest and post-harvest insect pest of sesame in North western Ethiopia. The rearing and other tests were conducted in the laboratory at 28-330C and 50 to 65% RH. The result of this laboratory investigation indicated that the mean egg incubation period was 5.4± 0.05 days while the total mean nymphal duration took 18.44±0.10. A mean adult lifespan of 20.24±0.19 days was recorded. The mean lifespan from egg to adult death was 44.08 days, which range from 32-54 days. The reproductive period was 7.08 ± 0.26 days and the peak laying period was on the first and second day of its reproductive days. Even though E.sordidus was present the whole year high outbreak was recorded at harvest (October) and continued up to January, which afterward declined until the next harvest time.