Fleas on dogs are tiny parasites typically about 1/8 inch (around 2.5 mm) long, though their size can range from 1 to 3 millimeters depending on the species and life stage. They are laterally flattened, wingless insects with six legs, and their body color varies from brownish-black to reddish-brown, especially when they are engorged with blood. Their flat body shape helps them move easily through a dog’s fur, and their powerful back legs enable them to jump great distances relative to their size. Adult dog fleas tend to measure around 1-2.5 mm in length and are often difficult to spot individually because of their small size and quick movement. Fleas commonly are found in thick, warm, and moist areas on a dog such as the base of the tail, around the neck, and in the groin area. Besides physical size, you might also notice flea dirt, which are tiny black or brown specks of digested blood in the fur, confirming their presence. In summary, dog fleas are very small, about 1/8 inch (2.5 mm) long, flat, and reddish to brown in color, making them hard to see but identifiable by their distinctive size, shape, and behavior on your dog’s coat.