Passport holders from Algeria, Côte d'Ivoire, Gambia, Libya, Mali, Niger, Senegal and Tunisia can visit for up to 90 days without a visa.
Visas are required by everyone else and cost around US$120 for a 3 month visit and take about a week to be issued. Visas are sometimes available at the border for 30 euros. They may only be valid for a few days.
Visa Information for Mauritania
