The Division of Sport Fish conducted studies in 1998 of chinook salmon Oncorhynchus tschawytscha in the Blossom and Keta rivers to estimate the number of large-sized spawning salmon, to determine expansion factors for aerial survey counts, and to obtain age, length, and sex composition of populations. The studies were conducted under agreement with the Joint Technical Committee of the Pacific Salmon Commission with Federal funding obtained to implement the 1996 U.S. Letter of Agreement between U.S. commissioners of the Pacific Salmon Commission. Escapement of chinook salmon was estimated using a two-event mark-recapture method. Fish were captured with set gillnet and rod and reel angling gear in June and July and marked with numbered spaghetti tags, an opercle punch, and removal of the left axillary appendage. Spawning and pre-spawning fish were captured later with angling gear to complete the experiments. The estimated escapements of large (greater than 660 mm MEF) chinook salmon were 364 (SE = 77) in the Blossom River and 446 (SE = 50) in the Keta River. An estimated 108 (SE = 33) medium-sized chinook salmon (440659 mm MEF) also escaped into the Keta River. Expansion factors calculated from the proportion of fish counted during peak aerial surveys to the number of large fish estimated from the experiments were 4.0 (SE = 0.85) for the Blossom River and 2.5 (SE = 0.28) for the Keta River. Previously, 2.5 was used for both systems. The dominant age classes for large fish in the Blossom River were age 1.2 (13.8%), 1.3 (33.0%), and 1.4 (38.5%) for both sexes combined. For large fish in the Keta River, dominant age classes were age 1.2 (10.6%), 1.3 (27.5%), and 1.4 (51.9%) for both sexes combined. Brood years from 1991 through 1996 were represented between both systems, with 13 age classes across all fish sampled. Age-0. (subyearling smolt type) fish composed 9.0% of all fish sampled from both systems.