Highlights:-Environmental impact of OMWW-Acclimated earthworms and bioconversion potential of OMWW-The high OMW ratio cause high mortality rate of earthworm 1. Abstract A laboratory vermicomposting (two months duration) was conducted to evaluate the effect of earthworm acclimatization (epigeic—Eisenia andrei) for decomposition and detoxification of olive mille wastewaters. The Earthworm growth rate and phenols reduction were superior in the mixture with acclimated earthworms (21.83 ±0.01 mg worm-1 day-1 and 72 % respectively), than mixture with unacclimated earthworms (4.67 ±0.01 mg worm-1 day-1 and 41% respectively). The data reveals that acclimatization of earthworms is a promising alternative for OMWW detoxification and vermicomposting (using E. andrei) is a suitable technology for olive by-products conversion into value-added material. 2. Introduction In Morocco, olive production was estimated at about 16 million tons of olive and 3 million tons of olive oil (Lakhtar and Roussos, 2016). The extraction of olive oil generates huge quantities of wastes that may have a great impact on land and water environments because of their high phytotoxicity. Therefore, there is a need for guidelines to manage these wastes through technologies that minimize their environmental impact and lead to a sustainable use of resources. Although various physical, chemical and microbiological methods of disposal of organic solid wastes are currently in use, these methods are time consuming and involve high costs. Therefore, there is a pressing need to find out cost-effective alternative method of shorter duration particularly suited to Moroccan conditions. In this regard, the viability of using vermicomposting technique with cost-effective to stabilize and manage the organic solid wastes. The subsequent application of vermicompost to soil demonstrates that they can be used as organic amendments to promote plant growth and regenerate degraded soils (Melgar et al,. 2009). The aim of the present study was to test the effect of acclimatization of earthworms to OMWW on their potential in vermicomposting at the laboratory scale, and to test earthworm's potential for OMWW detoxification after and before acclimatization as a means of recycling and treating olive by-product (OMWW and OP). 3. Material and Methods Two groups of earthworms (with clitellum) belonging to the species Eisenia andrei (Bouché, 1972), the first groups of earthworms were acclimatized to OMWW absorbed on olive-pomace, horse manure and wheat straw. The second groups of earthworms were bred in the same mixture without OMWW. Mixture composed of olive-pomace (60% D.W), horse manure (30% D.W) and wheat straw (10% D.W) were made. Olive mill wastewater (OMWW) was added to the mixture then pre-composted for one month. The vermicomposting study of olive oil by-products on a laboratory scale was carried out in 2.5 l plastic box (1 kg of the mixture) and inoculated with 5 adult earthworms (with clitellum). Two mixtures were prepared; the first one was inoculated with acclimated earthworms and the second one was inoculated with unacclimated earthworms. Each treatment was run in triplicate. The earthworm growth and reproduction were measured in each mixture. On the basis of the obtained data about the biomass, other parameters of earthworm such as biomass increase rate (mg day-1) and maximum weight achieved were produced with the help of the recorded data for different mixtures.