NOVELTY - Producing methacrylic acid and/or its derivative, involves: (a) producing mesaconic acid, citraconic acid, citramalic acid or its salt in a microorganism; and (b) decarboxylating and optionally dehydrating mesaconic acid, citraconic acid or citramalic acid produced in step (a) to methacrylic acid and/or its derivative. The step (b) is performed outside the microorganism to avoid significant toxic exposure of the microorganism to methacrylic acid and/or its derivative.    USE - The process is useful for producing methacrylic acid and/or its derivative, which is useful for producing methacrylic acid esters, where the methacrylic acid or methacrylic acid ester is useful for producing polymers or copolymers of methacrylic acid or methacrylic acid ester, including polymethacrylic acid, polymethyl methacrylate and polybutyl methacrylate homopolymers or copolymers (all claimed).    ADVANTAGE - The process enables the in vivo production of non-toxic precursor acids and the in vitro production of methacrylic acid using the precursor acids, thus reducing the wasteful destruction of cells, and allowing the processes to become commercial viable continuous processes, whilst still remaining environmentally friendly.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a microorganism which secretes or accumulates citraconic acid, mesaconic acid and/or citramalic acid;    (2) a fermentation process comprising culturing one or more microorganisms in a fermentation medium to produce citraconic acid, mesaconic acid and/or citramalic acid;    (3) a fermentation medium comprising one or more microorganisms;    (4) a bioreactor comprising one or more microorganisms;    (5) preparing polymers or copolymers of methacrylic acid or methacrylic acid esters, comprising preparation of methacrylic acid by the above process, optional esterification of the methacrylic acid to produce the methacrylic acid ester, and polymerization of the methacrylic acid and/or the methacrylic acid ester optionally with one or more comonomers to produce polymers or copolymers of methacrylic acid or methacrylic acid esters; and    (6) polymethacrylic acid, polymethyl methacrylate and polybutyl methacrylate homopolymers or copolymers formed from the above method.