NOVELTY - Converting mainly cellulosic material, comprises: e.g. (a) predisposing a quantity of mainly cellulosic material with a carbon/nitrogen ratio of greater than 50; (b) predisposing an anaerobic bacterial population; (c) predisposing a mixed aerobic bacterial population; (d) decomposition of the quantity of mainly cellulosic material into a liquid fraction, a gaseous fraction and a solid fraction by means of a predetermined thermochemical treatment; and (e) fermentation of at least the liquid fraction and gaseous fraction in a fermenter.    USE - The method is useful for converting mainly cellulosic material (claimed), which is useful for producing bioplastic.    ADVANTAGE - The method: is efficient and cost effective; utilizes waste from industry of the paper/offset printing; is rapid and efficient; enables reducing the energetic requirements; reduces the production of char; and does not comprise the passage of state (volatilization) of the products, thus can be performed on a moist or watery substrate (having a water content of up to 95 wt.%) with acceptable costs consumption.    DETAILED DESCRIPTION - Converting mainly cellulosic material, comprises: (a) predisposing a quantity of mainly cellulosic material with a carbon/nitrogen ratio of greater than 50; (b) predisposing an anaerobic bacterial population; (c) predisposing a mixed aerobic bacterial population; (d) decomposition of the quantity of mainly cellulosic material into a liquid fraction, a gaseous fraction and a solid fraction by means of a predetermined thermochemical treatment; (e) fermentation of at least the liquid fraction and gaseous fraction in a fermenter, by inoculating the anaerobic bacterial population in the liquid fraction and the gaseous fraction, for producing a solution (VFA) containing volatile fatty acids; (f) aerobic digestion of a first portion (VFAa) of the solution (VFA) by means of the action of the mixed aerobic bacterial population for producing a bacterial population enriched with polyhydroxyalkanoates; (g) aerobic digestion of a second portion (VFAb) of the solution (VFA) by means of the action of the enriched bacterial population for obtaining products rich in polyhydroxyalkanoates; and (h) separation of the products into a usable fraction (5a1), mainly composed of polyhydroxyalkanoates (PHA), and a residual fraction (5b, 5a2), at least partly composed of a cellular residue (5a2). An INDEPENDENT CLAIM is also included for a plant for the conversion of mainly cellulosic material with a carbon/nitrogen ratio of greater than 50, comprising: heating means provided with at least a chamber and designed to superheat the material contained in the chamber so as to break it down into a liquid fraction, a gaseous fraction and a solid fraction, where the chamber is provided with at least an infeed, for allowing entry of a predetermined quantity of mainly cellulosic material, and at least an outfeed, for allowing the liquid fraction and the gaseous fraction to exit the chamber; an anaerobic digester or fermenter operatively positioned downstream of the heating means, predisposed to receive the liquid fraction and gaseous fraction and designed to convert the fractions into a solution (VFA) containing volatile fatty acids by means of the action of an anaerobic bacterial population; a first aerobic reactor, operatively positioned downstream of the anaerobic digester, set up to receive a first portion (VFAa) of the solution (VFA) and designed to convert it, by means of the action of a mixed aerobic bacterial population, into a bacterial population enriched with polyhydroxyalkanoates; a second aerobic reactor provided with at least a first infeed and a second infeed respectively predisposed to receive a second portion (VFAb) of solution (VFA) from the anaerobic digester and the enriched bacterial population from the first aerobic reactor, where the second aerobic reactor is configured so as to make the second portion (VFAb) of the solution (VFA) react with the enriched bacterial population for obtaining products rich in PHA; and separating means operatively positioned downstream of the second aerobic reactor and configured so as to split the products into a usable fraction, mainly made up of PHA, and a residual fraction, at least partly made up of a cellular residue.