A series of criticality experiments with fast test reactor (FTR) fuel pins in water has been performed in support of the Advanced Fuel Recycle Program (AFRP). The objective of these experiments was to provide clean, easily defined criticality data on AFRP-type fuel pins in water for use in verifying calculational techniques and nuclear data used in calculations. Measurement data were obtained on water-flooded square lattices of FTR fuel pins. The number of fuel pins required for criticality was determined at lattice pitches of 7.7, 9.5, 9.7, 12.6, 15.3, and 19.1 mm to be 1268, 605, 580, 219, 205, and 162, respectively. These center-to-center fuel pin spacings correspond to water-to-fuel volume ratios of 1.61, 3.33, 3.49, 6.81, 10.88, and 17.53, respectively, and cover the neutron moderation range from near optimum to the highly undermoderated. KENO-IV calculations with ENDF data from the AMPX system overestimated the experimental results by 1 to 2% in keff. KENO-IV calculations with FLANGE-ETOG-THERMOS-EGGNIT-processed ENDF data resulted in calculated values 1 to 6% high in keff.