In the present investigation, we have studied the response of the ionospheric F-region in the Latin American sector during the intense geomagnetic storm of 21–22 Jan-uary 2005. This geomagnetic storm has been considered "anomalous" (minimum Dst reached −105 nT at 07:00 UT on 22 January) because the main storm phase occurred dur-ing the northward excursion of the B z component of inter-planetary magnetic fields (IMFs). The monthly mean F 10.7 solar flux for the month of January 2005 was 99.0 sfu. The F-region parameters observed by ionosondes at Ramey (RAM; 18.5 • N, 67.1 • W), Puerto Rico, Jicamarca (JIC; 12.0 • S, 76.8 • W), Peru, Manaus (MAN; 2.9 • S, 60.0 • W), and São José dos Campos (SJC; 23.2 • S, 45.9 • W), Brazil, during 21–22 January (geomagnetically disturbed) and 25 January (geomagnetically quiet) have been analyzed. Both JIC and MAN, the equatorial stations, show unusually rapid uplift-ing of the F-region peak heights (hpF2/hmF2) and a decrease in the NmF2 coincident with the time of storm sudden com-mencement (SSC). The observed variations in the F-region ionospheric parameters are compared with the TIMEGCM model run for 21–22 January and the model results show both similarities and differences from the observed results. Average GPS-TEC (21, 22 and 25 January) and phase fluc-tuations (21, 22, 25, 26 January) observed at Belem (BELE; 1.5 • S, 48.5 • W), Brasilia (BRAZ; 15.9 • S, 47.9 • W), Presi-dente Prudente (UEPP; 22.3 • S, 51.4 • W), and Porto Alegre (POAL; 30.1 • S, 51.1 • W), Brazil, are also presented. These Correspondence to: Y. Sahai (sahai@univap.br) GPS stations belong to the RBMC/IBGE network of Brazil. A few hours after the onset of the storm, large enhancements in the VTEC and NmF2 between about 20:00 and 24:00 UT on 21 January were observed at all the stations. However, the increase in VTEC was greatest at the near equatorial station (BELE) and enhancements in VTEC decreased with latitude. It should be pointed out that no phase fluctuations or spread-F were observed in the Latin American sector during the post-sunset pre-reversal time in the geomagnetic disturbance (21 January). The disturbance dynamo electric field possibly re-sulted in downward drift of the F-region plasma and inhibited the formation of spread-F.