Sikorsky UH-34D "Seahorse"

In 1955, the U. S. Marine Corps received its first HUS-1s as an interim type until the much larger HR2S heavy lift helicopter (later CH-37 Mojave) entered squadron service. However, the HUS lasted far longer in USMC service, and in much greater numbers, than the HR2S ever did. Ultimately the Marine Corps took delivery of 515 UH-34Ds. From the late 1950s until the CH-46 entered service in 1965, the UH-34 operated as the mainstay of Marine Corps helicopter units.

Vietnam

French evaluations on the reported ground fire vulnerabilities of the CH-34 may have influenced the U.S. Army's decision to deploy the CH-21 Shawnee to Vietnam instead of the CH-34, pending the introduction into widespread service of the Bell UH-1 Iroquois. U.S. Army H-34s did not participate in Vietnam, and did not fly in the assault helicopter role, however a quantity were supplied to the Army of the Republic of Vietnam. These saw little use due lack of spare parts and maintenance.

USMC UH-34Ds over Vietnam, 1965.The USMC continued to use the H-34 pattern, even after the U.S. Army had phased it out. Even after the USMC adopted their own version of the UH-1, the UH-1E, the CH-34s continued to be used up to and for a period after the Tet Offensive in 1968.

Its higher availability and reliability due to its simplicity compared to the newer helicopters led Marines to ask for it by name. The phrases "give me a HUS", "get me a HUS" and "cut me a HUS" entered the USMC vernacular, being used even after the type was no longer in use to mean "help me out".

USMC H-34s were also among the first gunship helicopters trialled in theatre, being fitted with the Temporary Kit-1 (TK-1), comprising two M60C machine guns and two 19 shot 2.75 inch rocket pods. The operations were met with mixed enthusiasm, and the armed H-34s, known as "Stingers" were quickly phased out. The TK-1 kit would form the basis of the TK-2 kit used on the UH-1E helicopters of the USMC.

On August 18, 1969, the last Marine UH-34D in Vietnam was retired from HMM-362 at Phu Bai. During that period, enemy action and accidents downed 134 helicopters. Most of the twenty surviving CH-34 helicopters were turned over to the South Vietnamese during the course of the war, though a few were ultimately reclaimed by the Army prior to the final collapse of the Saigon Government.

In the late 1950s, Air America, a CIA-created airline, began flying UH-34Ds in Laos, manned by crews on leave from the Marine Corps. When the last military UH-34 left Vietnam, Air America was still operating the type, including upgraded S-58Ts.

Post-Vietnam War

The H-34 remained in service with United States Army and Marine Corps aviation units well into the late 1960s, and was standard equipment in Marine Corps Reserve, Army Reserve and Army National Guard aviation units until replaced by the UH-1 Iroquois utility helicopter. Sikorsky production ceased in 1968, with 1,821 built. On 3 September 1973, the last flight of a USMC UH-34 occurred as bureau number 147191 was flown to MCAS New River. All H-34 helicopters were retired from service in the U.S. military by the early 1970s.