diff --git a/README.md b/README.md
index 5257b8d..77b42b7 100644
--- a/README.md
+++ b/README.md
@@ -29,3 +29,7 @@ The `values` property is a convenience function that provides a tuple of human-r
 * `temperature`:  the temperature in hundredths of a degree celsius. For example, the value 2534  indicates a temperature of 25.34 degrees.
 * `pressure`: the atmospheric pressure. This 32-bit value consists of 24 bits indicating the integer value, and 8 bits indicating the fractional value. To get a value in Pascals, divide the return value by 256. For example, a value of 24674867 indicates 96386.2Pa, or 963.862hPa.
 * `humidity`: the relative humidity. This 32-bit value consists of 22 bits indicating the integer value, and 10 bits indicating the fractional value. To get a value in %RH, divide the return value by 1024. For example, a value of 47445 indicates 46.333%RH.
+
+The BME280 constructor takes an optional `address` argument. BME280 devices are
+on I2C addresses 0x76 or 0x77. If you do not provide an address the driver will
+scan for a device and use it, raising an exception if no device is found.
diff --git a/bme280.py b/bme280.py
index 856f967..759824c 100644
--- a/bme280.py
+++ b/bme280.py
@@ -37,8 +37,8 @@ from ustruct import unpack, unpack_from
 from array import array
 from micropython import const
 
-# BME280 default address.
-BME280_I2CADDR = const(0x76)
+# BME280 default addresses.
+BME280_I2CADDRS = (0x76, 0x77)
 
 # Operating Modes
 BME280_OSAMPLE_1 = const(1)
@@ -55,23 +55,29 @@ class BME280:
 
     def __init__(self,
                  mode=BME280_OSAMPLE_1,
-                 address=BME280_I2CADDR,
+                 address=None,
                  i2c=None):
         # Check that mode is valid.
-        if mode not in range(1,6):
+        if mode < BME280_OSAMPLE_1 or mode > BME280_OSAMPLE_16:
             raise ValueError(
                 'Unexpected mode value {0}. Set mode to one of '
                 'BME280_OSAMPLE_1, BME280_OSAMPLE_2, BME280_OSAMPLE_4, '
                 'BME280_OSAMPLE_8 or BME280_OSAMPLE_16'.format(mode))
         self._mode = mode
-        self.address = address
         if i2c is None:
             raise ValueError('An I2C object is required.')
+        if address is not None:
+            self.address = address
+        else:
+            addr = [a for a in i2c.scan() if a in BME280_I2CADDRS]
+            if len(addr) == 0:
+                raise RuntimeError('No BME280 found.')
+            self.address = addr[0]  # 1st device found
         self.i2c = i2c
 
         # load calibration data
-        dig_88_a1 = self.i2c.readfrom_mem(self.address, 0x88, 26)
-        dig_e1_e7 = self.i2c.readfrom_mem(self.address, 0xE1, 7)
+        dig_88_a1 = i2c.readfrom_mem(self.address, 0x88, 26)
+        dig_e1_e7 = i2c.readfrom_mem(self.address, 0xE1, 7)
         self.dig_T1, self.dig_T2, self.dig_T3, self.dig_P1, \
             self.dig_P2, self.dig_P3, self.dig_P4, self.dig_P5, \
             self.dig_P6, self.dig_P7, self.dig_P8, self.dig_P9, \
@@ -86,7 +92,7 @@ class BME280:
 
         self.dig_H6 = unpack_from("<b", dig_e1_e7, 6)[0]
 
-        self.i2c.writeto_mem(self.address, BME280_REGISTER_CONTROL,
+        i2c.writeto_mem(self.address, BME280_REGISTER_CONTROL,
                              bytearray([0x3F]))
         self.t_fine = 0