What is the value of @serial you are providing. Update that in your question. Syntax to me looks correct.
For SQL:
For verification try this if its work fine:
DECLARE @serial varchar(10)
SET @serial = 'A'
SELECT serial, company_name, ..., ...
FROM (table)
WHERE serial like @serial + '%'
For Code:
string serialThatStartWith = "S";
string myQuery = string.format("SELECT serial, company_name FROM (Sometable)WHERE serial LIKE '{0}%'", serialThatStartWith);