The six original teams of 1905 were all based in Essex or Middlesex. Surrey was added when Dulwich and West Norwood joined the league in 1907, when there was also an expansion to distant Oxfordshire with the admission of Oxford City. Bromley became Kent's first Isthmian club a year later, Wycombe were Buckinghamshire's first representatives in 1921 and Hertfordshire was added to the list when St Albans joined two years later. After that, league membership was fairly stable and it was to be 50 years before further counties welcomed Isthmian football. Sussex (Horsham) and Berkshire (Maidenhead and Slough) staged their first games in 1973. Hampshire came as late as 1977, with Farnborough, and Barton provided a 1979 debut for Bedfordshire. Far-flung Somerset was next when Yeovil became members in 1985, after which there was an 18-year gap before Kettering provided what is, so far, a lone season of Isthmian action in Northamptonshire. A year later, in 2004, Salisbury's admission brought a one-off season in Wiltshire. Suffolk, now something of an Isthmian hotbed, had no members until 2006, when Bury and Sudbury joined. That was it until this season. Next? Perhaps Norfolk, with a promotion link from the Eastern Counties League now apparently firmly established.

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