To Richard
Trotsky did not belong to the Mensheviks.He joined the Bolshevik Party led by Lenin in the summer of 1917.

Trotsky was associated with the socialist grouping called Mezhraionstsy which had existed in Petrograd since 1913 and which kept itself independent of both the Bolsheviks and the Mensheviks.

The Mezhraionstsy joined the Bolsheviks in July 1917 bringing about 4000 members to join the Bolshevik Party which had about 400000 members. [ source" Trotsky :Towards October," Tony Cliff published by Bookmarks in 1989 ].
david