A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTERS OF SCIENCE IN CROP SCIENCE OF SOKOINE UNIVERSITY OF AGRICULTURE. MOROGORO, TANZANIA.Rice (Oryza sativa L.) is accepted globally as a major food crop. It is a staple food crop in many countries in Africa. There has been an increasing demand of rice in Africa. Africa consumes 11.6 million tonnes of rice per annum and out of 39 rice producing countries, 21 import 50 to 99 percent of their rice requirements. The inability to reach the yield potential that would sustain Africa’s need for rice is due to many biotic and abiotic constraints that rice production faces. In lowland grown rice, one of the abiotic factors hindering rice production is iron toxicity. Excess uptake of ferrous (Fe2+) ions leads to a physiological stress which results into poor production. The best way to control toxicity due to excess ferrous Iron uptake is by the use of tolerant varieties. The current study aimed at selection of varieties tolerant to iron toxicity and assessment of the genetic diversity linked to this trait. In a hydroponic experiment conducted in a screen house at Africa Rice Centre in Dar es Salaam, 32 rice varieties were evaluated for tolerance to iron toxicity. The experiment was laid out in a split plot design with iron concentration as the main plot factor and variety as the sub plot factor. Two levels of iron concentration were used: 2ppm and 300ppm of Fe2+ as control and test concentrations, respectively. Traits observed to gauge tolerance were leaf bronzing (an indicator of iron toxicity), plant height, tillering, number of leaves, shoot weight (above ground), root length and root weight. The varieties ARICA8, and CK801 were found to be tolerant due to low bronzing indices, higher shoot weight, more number of leaves and lack of significant variation in morphology between the two Fe treatments except for the plant height. Correlation analysis depicted negative correlation between leaf bronzing and the other traits measured especially shoot biomass. Assessment of genetic diversity was done using 12 SSR rice markers (RM). Out of these, 10 were polymorphic. There was an average 3.9 alleles per loci with an average polymorphic information content of 0.51. The most polymorphic locus was RM 341. Phylogenetic analysis revealed five clusters with ARICA 8 and CK 801 in two different clusters. In this study, 2 varieties showed tolerance, 22 were partially tolerant and eight were susceptible to iron toxicity. Genetic groups based on the molecular markers were related to rice species rather than iron toxicity tolerance levels.West Africa Agriculture Productivity program (WAAPP)