The Advanced Heavy Water Reactor (AHWR) is a new power reactor concept being developed at Bhabha Atomic Research Centre, Mumbai. The reactor retains many desirable features of the existing Pressurised Heavy Water Reactor (PHWR), while incorporating new, advanced safety features. The reactor aims to utilise the vast thorium resources available in India. The reactor core will use plutonium as the make-up fuel, while breeding ²³³U in situ. On account of this unique combination of fuel materials, the operational characteristics of the fuel as determined by its radioactivity, decay heat and radio-toxicity are being viewed with great interest. Radio-toxicity of the spent fuel is a measure of potential radiological hazard to the members of the public and also important from the ecological point of view. The radio-toxicity of the AHWR fuel is extremely high to start with, being ~ 10⁴ times that of the fresh natural U fuel used in a PHWR, and continues to remain relatively high during operation and subsequent cooling. A unique feature of this fuel is the peak observed in its radio-toxicity at ~ 10⁵ y of decay cooling. The delayed increase in fuel toxicity has been traced primarily to a build-up of ²²⁹Th, ²³⁰Th and ²²⁶Ra. This phenomenon has been observed earlier for thorium-based fuels and is confirmed For the AHWR fuel. This paper presents radio-toxicity data for AHWR spent fuel up to a period of 10⁶ y and the results are compared with the radio-toxicity of PHWR.