Certain relationships carry with them a fiduciary duty, or the duty to act with the 'utmost good faith and honesty.' Common examples are partnerships and certain professional relationships. There is also a fiduciary duty that runs between co-owners of a closely held (i.e., not publicly traded) company.
Generally, employment is not a fiduciary relationship, but there are exceptions:
If you are a high level employee in a position of trust and confidence, you may owe a fiduciary duty to your employer;
If you are both an employee and a minority shareholder in the company, assuming it is closely held, your other shareholders owe you a fiduciary duty even though as an employee you are employed at will.
The scope of these duties can be defined by contract- for example, if your operating or partnership agreement gives the majority unlimited discretion in terminating your employment, you may have difficulty proving a breach of fiduciary duty if you are terminated.
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A termination of a shareholder-employee for no valid reason, however, can in many circumstances be a breach of this duty, as the majority owners may be held to have unlawfully frozen you out of the benefits of your ownership in the company.