IN THIS PAPER, WE PRESENT THE EFFECTS OF PROXIMITYINTERFERENCE ON THE HYDRO-ELASTIC RESPONSES OF TWO PRETENSIONED FLEXIBLE CYLINDERS IN side-BY- side ARRANGEMENTSUBJECTED TO UNIFORM CROSS-FLOW (CF). THE EXPERIMENTALRESULTS OF THE side- BY-side CYLINDERS ARE PRESENTED ANDCOMPARED WITH AN ISOLATED FLEXIBLE CYLINDER. TWO FLEXIBLECYLINDERS OF THE SAME SIZE, PROPERTIES, AND PRETENSIONS WERETESTED WITH TWO DIFFERENT VERTICAL SEPARATION DISTANCES, NAMELY, 2.75 AND 5.5 DIAMETERS. REYNOLDS NUMBERS RANGEDFROM 1400 TO 20000 (SUBCRITICAL REGIME). THE ASPECT RATIOOF THE CYLINDERS WAS 162 (LENGTH OVER DIAMETER). THE MASSRATIO (CYLINDER’S MASS OVER MASS OF DISPLACED WATER) WAS1.17. IT WAS FOUND THAT AT THE SMALL SEPARATION DISTANCE OF2.75 DIAMETERS, THE UPPER CYLINDER SHOWED NO UPPER BRANCHIN THE FIRST CF LOCK-IN REGION, IN CONTRAST TO THE LOWER ANDTHE SINGLE CYLINDER. WITH THE INCREASE IN THE INITIAL VERTICALSEPARATION TO 5.5 DIAMETERS, BOTH CYLINDERS EXHIBITAMPLITUDE RESPONSES SIMILAR TO THE ISOLATED CYLINDER. AT THESMALL SEPARATION DISTANCE, THE UPPER CYLINDER SHOWED QUITEA DISTINCT CF FREQUENCY RESPONSE (HIGHER VALUES) THAN BOTHTHE LOWER CYLINDER AND THE SINGLE CYLINDER WITHIN THEREDUCED VELOCITY RANGE OF 4<U R<6.74.