A case of protothecosis causing non-ambulatory paraparesis in a dog without clinical evidence of disseminated infection is described. A five-year-old female Labrador retriever was referred with a 10-day history of progressive non-ambulatory paraparesis and lumbar pain as the only physical and neurological abnormalities. Lumbar myelography revealed severe extradural spinal cord compression extending from L4 to L7 vertebrae, and a right hemilaminectomy was performed. Surgical findings included an adherent whitish hard ill-defined mass. Cytology and biopsy results disclosed the presence of algae enclosed in a matrix of chronic inflammatory infiltrate. Culture confirmed the presence of Prototheca species. Neurological improvement occurred within a month, and the dog received antifungal treatment without evidence of clinical disseminated disease for 6 months, but died after a generalised tonic-clonic seizure. Post-mortem examination revealed multiple foci of inflammatory granulomatous infiltrate and algae-like structures in the brain, lumbar intumescence and cauda equina. Prototheca zopfii was identified using molecular biology methods. © 2014 British Small Animal Veterinary Association.