Biodegradable bottle cap is a water bottle cap containing the plastic eating fungus pestalotiopsismicrosporia. When the consumer is done drinking from the bottle, the cap has a tab that is pulled and this tab releases the fungus. The fungus consumes the plastic bottle and the cap so no trace is left behind. The cap is made of plastic and the fungus is contained in an aluminum casing. Also contain in the casing is the safe food dye which will be coloring the water in case of accidental release or opening of the casing before the use of the drinking water. Such water coloring will be an indication to avoid using the water for drinking though the fungus is safe after human galloping.