Prosopis juliflora is an invasive plant with a negative allelopathic effect on several native caatinga species. Current investigation verifies the allelopathic effect of leaf extracts and seeds of algarroba on lettuce germination. Extract for each plant section was obtained at 25°C and 100°C. Randomized design was employed with five treatments made up of extract concentrations by dilution with distilled water (0, 25, 50, 75 and 100%), with four replications of 25 lettuce seeds each. Extract of fresh leaves of P. juliflora obtained with water at 25°C failed to affect lettuce germination, varying between 92.5 and 100%. Extract of dry leaves of P. juliflora at 25°C decreased lettuce germination in greater concentrations. Extracts of fresh and dry leaves of P. juliflora at 100°C reduced germination in most concentrations and the index of allelopathic effect is negative. Extract of P. juliflora seeds at the two temperatures negatively affect germination, mean germination time, length of the lettuce´s aerial section and root, with abnormal plantlets. The use of bio-herbicides from Prosopis juliflora may be interesting in family agriculture and agroecology. More research should be done in this area with the identification of compounds causing such effects.