High Point of League Expansion

The league in 1951-52 expanded to 11 teams, with such new teams as the Los Angeles Fibber McGee & Mollys, Artesia REA Travelers, and Santa Maria Golden Dukes. The 66ers just edged the Oakland Atlas-Pacific Engineers and the San Francisco Stewart Chevrolets for their fourth title, with a 17–5 record to their opponents 16-6 records that tied for second. The next season (1952-1953), the league dropped down to nine teams, but saw new opponents in the Houston Ada Oilers and the Los Angeles Kirby's Shoes. The 66ers edged the Caterpillars for the title by one game, with a 13–3 record.

The Peoria Cats tied the 66ers for the 1953-54 title, each with a 10–4 record. The next two seasons, the 66ers and the Cats took first and second respectively. A new team in the greatly reduced circuit of five teams in 1955-56 was the Wichita Vickers.

The 1956–57 season was one of the most competitive in the NIBL history. While the 66ers again took first with a 13–7 record, four teams tied for second with 11-9 records, including the newly added Denver-Chicago Truckers. This proved to be the last season for the Milwaukee Allen-Bradleys, who finished last for the seventh time with a 3–17 record.

The 1957–58 season saw the Vickers move to the forefront, tying the 66ers for the league title with a 21–9 record. A new team that year was the Kansas City Kaycee's.

End of the Phillips 66ers Winning Streak

In the 1958–59 season, the 66ers failed to take the league title for the first time since their coming to the league, finishing in third place. The Truckers finished first with a 21–9 record, with the Vickers ruunerup at 19–11. Joining the league that season was the Seattle Buchan Bakers. However, in the 1959–60 season, the 66ers regained their title as league champions.

Demise of the NIBL

By the early 1960s, NIBL teams were increasingly struggling to comepte with the salaries offered in professional league. Top college graduates increasingly gravitated to the NBA, and the NIBL began to decline in popularity and profitability. This is evidenced by the disbandment of the Peoria Cats at the end of the 1959–60 season.

In the NIBL's final season, 1960–61, the league had dropped down to only six members divided into two divisions, Eastern (Cleveland Pipers, Akron Goodyears, New York Tuck Tapers) and Western (Denver-Chicago Truckers, Phillips 66ers, and Seattle Buchan Bakers). Instead of the round-robin schedule determining a winner, the league sponsored a four-team playoff. The Pipers beat the Truckers for the championship, 136–100. The 66ers beat the Goodyears for in a match for third place, 114–112.

In 1961, the league dropped their industrial sponsors and merged with other amateur leagues to form the National Alliance of Basketball Leagues (NABL).^[1] The Pipers and the Tapers left to join the newly formed American Basketball League in 1961.