Vanguard Group and Fidelity Investments manage hundreds ofbillions of dollars in defined-contribution money for thousands ofretirement plans with millions of participants. Against competitionlike that, a foray into 401(k) plan management by a unit ofrobo-adviser Betterment, a company with $6.2 billion in assetsunder management, seems quixotic, to say the least.

Betterment for Business, which began signing up corporateclients in January, announced today that it has more than 300companies on board, with plan assets ranging fromhundreds of thousands of dollars to more than $25 million. (Itdoesn't break out assets by business unit and won't give anumber for total assets in the 401(k) unit.) Clients range fromtechnology companies such as project management softwaredesigner Trello to mattress maker Casper and midsizeprofessional service businesses such as law andarchitecture firms, as well as medical and dental practices,Betterment said.

It may be good timing. The Department of Labor's fiduciary rule,which takes effect in April, requires advisers for retirementinvestments to put a client's interest before their own. A2015 government report estimated that adviser conflictsof interest cost investors some $17 billion a year. The rule couldbe a regulatory catalyst for growth, said Tyler Cloherty,senior manager with Casey Quirk by Deloitte.